ixbrl tax computation

I have been using HMRC's software for submitting CT returns online (I have only done three, but no problems experienced to date).  I now have a client with a loan from an asociated company written off, and there appears to be no facility to make this tax adjustment on HMRC's tax comp pages.  Does this mean I need to spend £100s to buy commercial ixbrl software just so I can submit this client's tax comp, or does anyone know of a feasible workaround.  I don't need the accounts preparation software anyway (as I have done that on HMRC's site), but I am not sure whether I can get just the tax comp.  I have tried looking at some of the software sites (tax calc, sage etc), but all seem to require significant investment of cash.

I would be grateful of any help or advice, thank you.