if not what can i use

Heat pads, just google and you will find various options.  Many here like the vet nap ones.

An electric blanket would be dangerous as the pups will wee through their bedding onto it, and how would you wash it. 

I have used the flat sealed vivarium ones to goo effect too, as nto so bulky, but mine don't' need a lot of heat.

Try here:
http://cgi.ebay.co.uk/Pet-bed-heater-Cat-dog-heated-Petnap-Warmer-Pad-F33-/350422250519?pt=UK_Pet_Supplies_Cats&var=&hash=item905b6dd6ff

http://cgi.ebay.co.uk/Cat-dog-bed-pet-puppy-electric-heat-pad-heater-mat-SAFE-/350461866012?pt=UK_Pet_Supplies_Dogs&hash=item519927b01c

Ms Temeraire has shown you a really very good example - but you can buy diretc from the manufacturer more cheaply.  The only link I have has the metal one 2nd photo down.  Once I had used my first, I gave away my vinyl one, and would never go back.  Even the cable is chew proof.  Not the cheapest, but will last forever.

http://www.petnap.co.uk/acatalog/pet_heat_pads.html#a30

> Ms Temeraire has shown you a really very good example

Thank you Jean :) oddly enough, the links are from the manufacturer! They sell via ebay as well.
My first link was the metal pad, the second was the vinyl type. I've had both in the past (for kittens) and agree that the metals ones are better especially with the cable covering.