Sorry for being a bore, but I can't find what I am looking for though I'm sure I've seen this question many times before.
I am looking to get a new heat pad for the whelping box. Any suggestions (or link to previous post where this has been asked before!) gratefully received.

Google Petnap.  It's a metal pad.

I have found the soft vinyl types useless as found they came unglued at the seams from the heat/humidity in the whelping box.

Definitely the metal Petnap with the metal covered electric lead.

I have one of the plastic microwaveable ones and it’s ok if you need it as a temporary portable one but doesn’t hold the heat well enough for permanent use.

As Brainless says, if you Google ‘Petnap heatpad’ you’ll find the site to order from.

Thanks guys
I have been looking at both the vinyl and metal Petnap ones. I had decided on the vinyl one, so glad that I asked.
If the metal one became uncovered, I assume it is not hot enough to burn a pup?

I don’t think it would burn a puppy, even uncovered, I bought a couple of the covers for it and they fit really snuggly and haven’t ever come off.

None of them are hot enough to burn a pup, that would be dangerous.  They all provide a very gentle heat.  I rarely need heat pads though, and find they often create problems because the pups are drawn to the warmth and the mother is often too hot and so places herself elsewhere in the box, so the pups are not with mum and nursing, they get confused and end up on the heat pad - because it's an alternative heat source.

I have large breed, and I've always used the vinyl one (and have a couple of spare covers for washing).  I've found it excellent, just have to be vigilant at first to make sure pups don't go under it and get stuck.