I've worked out the taxable gain based on your occupation record, a 50% share of the gain and the property not having been let. If you exchange contracts to sell the property after 5 April 2014 for £600,000, having paid £324,000, you will make a gain of £276,000, £138,000 for each of you and your former partner. By that time you will have owned it for 87 months of which you will have lived in it for 49 and not lived in it for 38. The following figures represent your tax position based on your record of occupation of the property, not your former partner's:
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 will account for £106,276 (£138,000 / 87 x 67). The remaining taxable gain of £31,724 is that part of the gain when you were not living in the property that is not covered by the last 18 months of ownership (£138,000 / 87 x 20).
The annual CGT exemption for 2014/15 of £11,000 will reduce the remaining taxable gain from £31,724 to £20,724. On a salary of £45,000, all the taxable gain of £20,724 will be charged to CGT at 28% leaving you with a CGT liability of £5,803.
If you can exchange contracts to sell by 5 April 2014, you will reduce the taxable gain by £11,103 (£138,000 / 87 x 7). This is based on April 2014 being 25 months since you moved out of the property. If you sell in the current tax year, your CGT will be payable on 31 January 2015 as opposed to 31 January 2016 for a 2014/15 disposal.
I hope this helps but let me know if you have any further questions.