Apart from the possible inheritance tax, there's nothing to worry about. As long as the house is your main home, Capital Gains Tax does not apply:
http://www.direct.gov.uk/en/MoneyTaxAndBenefit s/Taxes/TaxOnPropertyAndRentalIncome/DG_401633 7
Gifts are not taxable (except when IHT applies). If someone wishes to give you �230,000, they're free to do so, without any financial penalties to either them or you. If you then choose to use that money to pay off your mortgage, you'll then be using your own money, so the situation is just the same as if the money came from your savings.
Chris
PS: When you, or the relative pays the mortgage off, remember to keep the receipt. I received a 'notice of intent to repossess' several months after I'd paid off my mortgage, because the Woolwich had managed to 'lose' my payment of over �35,000! Thankfully, I'd still got the receipt, so I managed to get things sorted out very easily.