I bought my house in July 1994 for £138,000 and lived in it until May 2004 and after doing it up with an interest only mortgage of £80,000 I managed to let it from July 2004 until now (June 2012). I would like to sell it (agents valued it at around £485,000 - less costs of agents and solicitors of about £10,000 incl VAT).I would like to pay off the mortgage and buy a cheaper buy-to-let flat for about £150,000-£200,000 without borrowing, and spend the rest on some enjoyment and help the children with deposits on their own homes.

Your initial capital gain is £(485 - 138)K = £347K. On the assumption that you spent  £80K on capital expenditure on the house, and you have allowable incidental costs of £10K, your net capital gain is now £257K. You have owned the house for 18 years, so the gain per year is £14.27K. 

The first 9 years 10 months are exempt, due to actual occupation as your principal private residence, and the last 3 years of ownership are also exempt due to deemed occupation. This reduces the gain by £183.23K to £73.77K. Between Jul 04 and Jul 09 (i.e. the date 3 years before assumed sale) you rented out the property so you can use the letting exemption to reduce the remaining gain by a further £40K to £33.77K. 

Assuming that you have the capital gains tax annual exemption of £10.6K available to you, your final taxable gain is £23.17K. This capital gain is added on to your income, and any part of the gain falling below the higher rate threshold of £34,370 will be taxable at 18%, and any part above will be taxable at 28%.