Is it possible to have mastitis in one teat and the others all OK?

Yes.

and so the milk from the other teats will be OK but not the one?I am still working with hot compress.
It appeared to be fine but has returned. Will get to vet but wondered about the milk the puppies have been taking?

Keep latching the puppies on to all the teats, the affected teat needs to clear through to get back to proper milk. Antibiotics will help...

Yes it's the only way with both cases I have had that it presented 

The first was boxing day 2006, and at 10 days.

Strong AB's (Noraclav) and drawing off infected milk the colour of tomatoes soup, going to mushroom, finally to normal.

Pups fed from Mum throughout, avoiding the infected boob until all infection gone and normalk milk produced.

This year similar situation, pups older.  Same treatment pups stayed on Mum, but this time could not draw anything from infected breast, and afterbtwo days it ruptured, and puss squeezed out and wound cleaned with salt water.

Continued to nurse happily (pups running around by then) and after all contents drained the gland healed very quickly, before AB course finished.

Interestingly in both cases no problem pups avoiding the infected gland.

Ok thank you bit of a torrid time in the last hour. Pups have fed in groups being kept away from the problem teat.
I have worked and worked with hot compress and finally the hardness in the one seems to have subsided. She does not enjoy me doing this and I worry whether the water is too hot.
This is not the teat that is a problem with the milk but the one in front of it but I have tried and tried to get the bigger pups to use the back ones but they favour others.
The smallest puppy is completely happy on the back teats.
The one problem one sometimes seems to have white milk sometimes a bit mushroomy as if it is somewhere between going right or the other way.
It has never been tomato coloured and still sometimes milky white so I am hoping with reducing the pressure it will save the situation. 
I think I will continue the compressing this time until I am sure it has disapated.
She seems very happy in herself ( although I am keeping a watchful eye for any change) and the only thing is she seemed slightly distressed that some were kept in a box away from her.

The pups will avoid it if the milk is bad, but will feed if it's OK. So no special action needed to keep them off if it goes bad.

Keep the pups with her and they will avoid the "bad" teat, compresses need to be comfortably warm to your inner arm, they don't need to be hot, hopefully this will have resolved in a couple of days.

Things appear to have cleared through after the effort last night. The milk from the teat is now the milky white.
I am not totally convinced the one behind has totally gone back to normal so am just getting an appointment now to have her checked out.
Have been leaving her with the pups to feed normally.

As you've already been told, yes, it's quite possible (and more likely I'd suggest) for just one teat to be affected.   It's happened with us.   You just have to be there to keep the puppies off the affected teat, treating with warm compresses (you may be doing this with the water too hot) and probably with antibiotics too.    If any of my girls were needing ABs, I'd give the puppies a tiny amount of natural yogurt to counter any effect mum having ABs might have.

You might consider cutting back on the protein mum is having if she's producing more milk than the puppies can draw off?   But as I always did, be there when they are feeding, to make sure all the teats are being drained during the feeding.

I have never had need to use this remedy myself but I have been led to believe that warm cabbage leaf compresses are very good for mastitis.

Yep it works well for humans so.ahpild.be the same for.dogs

An update then. I am not an inexperienced breeder by the way but have never seen this mushroomy colour.
Overnight I stayed with her when feeding but to be honest the teat seemed to have freed up and by now was producing milky white again.
I decided to go to the vet who gave her a good examination. She deemed she did not have mastitis and temperature was normal but I was surprised to find that when going through each teat, a different one on the other side which had not had any hardness at all, had this colour but then changed to milky white.
She opted against antibiotics at this stage as she felt there was nothing particularly unusual in what she found. Unfortunately my favourite vet is on holiday this week so don't know about her experience but at the current time all the teats are producing milk white liquid. We discussed whether because the one the other side had become really quite hardened, ( now freed up) it had caused a blockage.
I will just have to keep a good eye on her until weaning but I'm a lot happier than I was last night.

Congestion can be a precursor to mastitis, and can be avoided by due diligence as you have done. I had to work hard on a bitch last year for several days, but managed to avoid it.

Once infection sets in that is true mastitis as in the cases I wrote of. Both came on very quickly and the bitches became unwell. Human sufferers tell me it is like flu, and there will be a temperature.

The colour of the discharging liquid will depend on how bad the infection is and may contain blood puss or and/or spoilt milk.