helping grown up children with purchase of house

If you live more than seven years after giving the money, it will not be part of your estate for inheritance tax.
In any event, it would only be taxable if your estate was above the threshold for inheritance tax.
Try googling 'Valuing life time gifts for inheritance tax' and you will get the appropriate HMRC site - please do not rely on information from anyone on any forum (even retired tax inspectors!)

The only other issue I can think of is the need for care on the cards.
I know somebody who lived in a Council Property and his son worked in London.
The son got early retirement and sadly his mother passed away within days.
The son sold his house and purchased a bungalow near where his father lived and his father moved in and lived there for 9 years.
The father then had to go into care and the Social Worker said that half the bungalow belonged to the father even though the son had paid for it and he was the sole owner of the property.
Fortunately the son was able to prove he had sold his house in London and part of the proceeds were used to pay for the bungalow.

Martin

Martin, I used to be a conveyancer and I know of no law that says that a relative moving in with a house owner becomes a part-owner of the house! Social Workers are probably not experts in property law.
If the father had contributed to the cost of the house, his share should have been registered with the Land Registry. If he had made a gift of the money to help his son buy the house, I think that would have counted as a life-time gift and my first reply would be relevant.
I gave both my daughters a considerable sum (£20,000 each) as deposit on their houses, but I did not want my name on the deeds. I was only 42 and in good health, and in any case I had no capital left. My employer was providing me with accommodation abroad, so I used my half share of the matrimonial home to get my girls on the property ladder. (Unfortunately, when I returned to England I had to start again from scratch with a mortgage!)

Much will depend on your circumstances. Make sure it is recorded as a gift at the time of giving so that any lifetime gift clock starts at the correct time. Have you thought about lending him the money - there is then a debt due to your estate which you could forgo in your Will? Speak to a probate/tax solicitor who will be able to tell you the best way of dealing with it. Also if you had any other children, you could ensure that things were 'equalised' if that is what you want to happen. Questions may be asked about the origins of the money for money laundering purposes if your son is taking out a mortgage - also his lender will want to know whether you have any interest in the property or the proceeds of sale - again this is something on which a solicitor can advise.