We currently have a home valued around £150,000 (no mortgage) and in the not–too-distant future we are looking at renting a bungalow via a housing association.  Our son has a mortgage of £48,000 on a small ex-council house.  He suffers with M.E. and had to give up one day a week at work due to this.  We would like him to have our home as it is detached but can't afford to just give him it as we will need money for rent. We would like to sell it to him for what he can afford which would be about £55,000.  Could he sell his home and then get a mortgage on ours without any problems arising?

You can sell your home to your son for what he can afford without any immediate tax concerns. I presume that the house is your principal private residence. If so there is no capital gains tax when you sell. Since your son is only paying you £55,000 for the house, there will be no stamp duty land tax on his purchase. Since this is a transfer between connected persons (parents and child), he is deemed, for future capital gains tax purposes, to be purchasing the house at present market value, i.e. £150,000.