Hi. My name is*****'m looking at your question now and will post my answer or ask for more information here in a short while.
You should read the notes here and here. You need to report your gain to HMRC within 30 days of the completion of the sale of the property.
The information in the second link above gives you three ways of calculating the gain for CGT purposes. You will probably be best off choosing to use the 5 April 2015 value of the property as the "cost" for CGT purposes as opposed to the price you paid in 1998. If you do, then only the increase in value since 5 April 2015 will be chargeable to CGT. The first £11,100 of your gain will be tax free due to the annual CGT exemption. Your UK income is also used in assessing how much CGT you pay.
Us the calculator here to estimate your UK CGT liability. It will ask you for information so that it can assess your CGT. If the property was ever your main home, then you will pay less CGT because of that.
I hope this helps but let me know if you have any further questions.
Hi.I'm just following up to find out if my answer helped or if you have any further questions.