My [medium size] bitch had a large litter of pups on Sunday (second litter).  She will not leave them apart from very brief visits for the toilet, before stressing to be let back into them.  I am having to present her food and water to her in the whelping box in order to get her to eat and drink and she will not let anybody near the pups.  I am weighing the pups when she is out the box for the toilet and they seem to be doing well.
She seems exhausted already, probably not having rested at all since the birth, and refusing the puppy food that she had been eating beforehand so I am giving her a variety of food to tempt her and she is loving the tinned dog meat.
She obviously needs to give herself a break but just stresses when she is away from them, so how do I get her to rest away from them?
Any advice gratefully received.

This is 100% normal behaviour from a newly-whelped bitch. I wouldn't expect a bitch to be willing to leave her pups for at least 5 days, needing to be put on collar and lead and forced out to toilet several times a day, and be fed - fluids more important than solid food at the moment, especially if she ate the afterbirths - in her whelping box. Leave it to her to decide when she's ready to leave them!

She'll be getting plenty of rest, but is possibly stressed and uncomfortable as her uterus contracts and her milk starts to come in.

Ok.   Most of what you are saying sounds pretty normal to me other than not letting anybody near the puppies, especially you?   If she's had puppies before, was she the same because quite honestly if any of mine acted like that, one litter would have been it.  I'd not expect her to be happy for all and sundry coming around yet however.  

Eating in the whelping box - fine.  Water can be placed outside so when she does go out, she can 'tank up' before going back to the litter.   And early days mine have sometimes had to be coaxed out on a lead to attend to biz.  At that point I'd do a puppy weighing, and general clean up of the whelping box before letting her back in with them  

She does NOT need 'puppy food'.   I have no idea when all this started, but again (and again) puppy food is for puppies to eat, not adults/mum.  She should be on the same top qualify food she always eats, with, as she has a big litter, more of the same + more protein - chicken etc.

Whether or not she needs 'a break is debatable.   You may think she does, but if she's only newly whelped, after being sure she's empty, she just needs peace and quiet to rest with her new brood, with you keeping an eye on proceedings.   This is the time she should be doing all the work - until you begin weaning them.   Don't make her rest away from them.  She wants to be with them, and quite rightly at this point.

If she's restless and won't settle, you might consider whether she has some retained whelping material which needs to come out.  Again if she's had a litter before, you should know that?

Thank you. She did pause for a little longer on her last toilet break, and had a little cuddle with me, so will just carry on as we are.
She did the same with her first litter but just a little more worried this larger litter is taking more out of her.

This is totally normal and actually a sign of a good mum.  I'd be a bit worried if it were anything other than how you describe...

Gosh you expect a new mother to be happy away from her puppies, all female(not just dogs)animals want their new borns near them, instinctive behaviour necessary for successful rearing. She will rest whilst the puppies sleep & regular suckling is needed to encourage the milk flow & prevent mastitis.