Your accountant should refer to HS283 for information on the main home and CGT.
I'm assuming you sold the property in December 2015, not December 2016 and that it was let to the point of sale. The gain was £65,000 (£245,000 - £145,000). You can also deduct the costs of purchase and sale (legal fees, stamp duty, survey fees, selling agent fees).
Total period of ownership to December 2015: 152 months
Period of owner occupation: 32 months
Period of letting: 120 months
Exempt gain: £21,382 (£65,000/152 x 50 (32 months + last 18 months of ownership)
Letting period gain: £43,618 (£65,000/152 x 102 months (152 - last 18 months of ownership)
Gross Non-Exempt Gain: £43,618
Letting Relief: £21,382 (lesser of £40,000, £21,382 and £43,618)
Annual CGT Exemption: £11,000
Net Taxable Gain: £11,236
CGT @ 18% or 28% or a combination of the two rates depending on the level of your income in 2015/16. Look here to see how to calculate your CGT rate. Based on what you told me, you will pay CGT at 18% (£2,022.48).