Completing CGT on joint owned property advice

Can someone advise me on the key things to do when adding the CGT extra form on to my self assessment. The self assessment phone line is closed until 4th September. I had a go and it said I owed a huge amount of tax on the CGT property, but I've paid what I own last September Do I have to declare it again and how can I do this without being asked to pay again. I've realised that I needed to half the costs as it's a jointly owned property but I'm really struggling with adding it to my tax return.

Hi Adrienne O'Sullivan,

If you have reported and paid the capital gains tax using the online service, you will need to declare it again on your self assessment tax return, only if both of the following statements are 'yes'.  
1 - the disposal value is more than £49200.00.
and
2 - You need to complete a self assessment tax return for any other reason.  
If you have already paid the capital gains tax, there are two additional boxes that need to show the gain and tax paid.  
They are name
"Total gains or losses on UK residential property reported on Capital Gains Tax UK Property Disposal returns"
and
"Tax on UK residential property gains reported on Capital Gains Tax UK Property Disposal returns already charged".  
By completing these boxes as well, you will be given a credit in the self assessment calcuation, for the tax already paid.
If you need further help, you can contact my colleagues using webchat at Self Assessment: general enquiries, 8am to 6pm Mon to Fri.

Thank you.