You made a gain of about £52,500 (£385,000 - £265,000 - £15,000 / 2) when the property was sold. As the property was jointly owned, the gain is split 50:50 even though you paid for the enhancements.
You are entitled to a deduction from the gain called taper relief which is determined by the number of complete years of your ownership of the property, this being seven in your case which entitles you to taper relief of 25%. The net gain is, therefore, £39,375 (£52,500 - £13,125). The annual CGT exemption of £8,800 reduces it further to £30,575.
A married couple can only have one main residence between them and unless your husband lived in the property with you whilst you were caring for your mother, I doubt that the tax office will allow any main residence exemption. That is unless you made an election for the property to be treated as your main home within two years of acquiring the property which I doubt you did as your mother was living there.
The net gain of £30,575 will be added to your income for 2006/07 in order to determine the tax rates to be applied to it. Take a look here
for the tax rates for 2006/07. The CGT charge will be between £6,486.50 (10% and 22%) and £12,230 (40%).
I hope this helps but let me know if you have any further questions.