E241

I have just sold a property which I owned for 10 years. For the first 3 years I lived in the property as my main residence. For the last 7 years, I let the property out and lived elsewhere. According to the worked example here (https://www.gov.uk/tax-sell-home/let-out-part-of-home) I would get Private Residence Relief for 3 years out of 10 (30%), plus the last 9 months of ownership. But when I finished living in the property, I tried to sell it. I had an offer accepted via an estate agents. The sale ended up falling through and I decided I could rent out the property instead. As a rough example, I bought the property for £330,000 and sold it for £380,000. The gain in value is £50,000 over 10 years. But the agreed offer when I tried to sell the property after 3 years was £370,000, so £40,000 of the gain occurred when I was living in the property as my main residence, and while I let the property out for the last 7 years the gain was only £10,000. Am I able to say that the gain I would need to pay Capital Gains Tax on is just £10,000 (i.e. the difference between £370,000 and £380,000) or do I need to take it as 30% of £50,000 (i.e. the difference between the original purchase price of £330,000 and the final sale price of £380,000). Thanks for your help.