If anyone has several dogs or animals and wants to get them microchipped for free it is worth checking out the college of animal welfare at http://www.caw.ac.uk/mainpage.asp
Check out 'what's on' and it has a list of when they do free microchipping at different locations.
We had 4 cats and one rabbit and it would have cost a lot to get them all done so it was well worth the journey up to get them done for free. The people there are trainees but they were all really professional and really nice, even though one of our cats was very ill on the way over, bless.
I would definitely recommend them as it helps to train people too and most of the people there are learning to work in rescue centers so it helps them too.
Otherwise friends of ours got their dog done for just £10 at the RSPCA tent at a charity dog show so they're worth checking out too.
Hope this helps