Michael, thank you for your reply..
The property has been let for 25 years (300 months). Say you own the property for a total of 35 years (420 months).
Your private residence relief would be for a period of (120+18)=138 months out of 420 months and the rest of the period falls within chargeable gain.
Say your total gain is £100k.
Gain covered by private residence relief138/420x100,000 = £32,857
Gain subject to CGT (100,000-32,857) = £67,143
Letting relief - max £40,000
Chargeable gain (67,143-40,000) = £27,143
Gains allowance = say £11,000
CGT payable on (27,143-11,000) = £16,143
If the property is jointly owned with your wife then maximum letting relief would be (40,000x2) = £80,000 and will cover gain after PRR resulting in no chargeable gain.
I hope this is helpful