The HMRC helpsheet HS283
will give you some useful information on the main residence and CGT.
As you did not make an election for the property given to you by your mother to be treated as your main home, that will now be determined by the facts but, given that you had separated from your husband, I would say it is clear that for the past four and a half years, the property you live in now has been your main home. This will have an impact on your share of any gain you may make if you sell the marital home at some point in the future.
You need to divide the gain (sale proceeds net of disposal expenses such as legal fees and selling agent fees less value in 1998 which is your "cost" for CGT purposes) by the number of months or days of ownership. The gain is treated as having accrued evenly over the entire period of ownership.
That part of the gain covered by your occupation of property will be exempt from CGT as will the gain for a maximum of the last 18 months of ownership if you were not living there during that period. That accounts for 4.5 years worth of the gain as of now, a little more if you stay there until it is sold in a few months. The remaining gain will be taxable but the first £11,000 of that will be tax free due to the annual CGT exemption.
There are two rates of CGT, 18% and 28%. The rate or combination of rates that you will pay will be dependent on the level of your income in the tax year of disposal. If you sell the property in the current tax year, 2014/15, one of the following scenarios will apply:
1 If your income in 2014/15 including the taxable gain is £41,865 or less, then all the taxable gain will be taxed at 18%.
2 If your income in 2014/15 excluding the taxable gain is more than £41,865, then all the taxable gain will be taxed at 28%.
3 If your income excluding the taxable gain is less than £41,865 but more than £41,865 when you include the taxable gain, then part of it will be taxed at 18% and part at 28%.
Finally, as your mother continued to live in the property after she gifted it to you, the gift will have been a gift with "reservation of benefit" which you can read about here
. Assuming she lived in the property until she passed away, its value at that time should have been included in her estate for Inheritance Tax purposes. Whether her estate would have been liable or not is something you need to consider as it may come to light at some point. You can read about Inheritance Tax and the transfer of an unused nil-rate IHT band here
I hope this helps but let me know if you have any further questions.