You should refer to HS283 here
for information on the main home and CGT, having read this answer.
First off, the value of the property when it was first let is irrelevant to the calculation of the gain which is computed on a straight line basis.
If you sell the property for £325,000 in August 2015 having paid £70,000 to buy it, you will make a gain of £255,000, £127,500 for each of you and your wife. Once you deduct the costs of buying and selling (legal fees, stamp duty, survey fees, selling agent fees etc), the gain for each of you will be lower. By August 2015, you will have owned it for 240 months of which you will have lived in it for 132 and let it for 108. The following will apply to each of you and your wife:
The gain for the period the property was your main home will be exempt from CGT as will the gain for the last 18 months of ownership. That accounts for £79,688 (£127,500 /240 months x 150 months). The remaining non-exempt gain is £47,812 and that covers the period the property will have been let apart from the last 18 months of ownership.
As the property has been both your main home and let, you are entitled to a further deduction called letting relief which is the lesser of:
2 the gain for the period that the property was your main home plus the gain for the last 18 months of ownership, £79,688 and
3 the gain for the letting period less that part which coincided with the last 18 months of ownership, £47,812.
Letting relief of £40,000 will reduce the remaining non-exempt gain to £7,812 and that will be covered by the annual CGT exemption of £11,100 which means that you will have no Capital Gains Tax to pay.
I hope this helps but let me know if you have any further questions.