Letter claims we only sent front and back page of SA800

We sent a paper Partnership return - SA800 - the full original document, to HMRC in October. It was marked online as received on 10th October. Subsequently, in late November, we had a letter from HMRC saying our return was invalid as we only sent the front and back pages. The letter included a scan of the front and back pages as 'evidence' and said we must now do the online form (which will cost us money). The 'evidence' scans clearly showed, from pages behind the scans of front and back, some of the other pages, with their marginal code identifying them as other parts of SA800. So the 'evidence' actually showed the whole document was sent. A complaint letter sent a month ago has had no response. What are our options? We sent the whole SA800. We are not newbies at this - we do it every year. Your scans support our case. We should not be being penalised (which we will be either witha fine or the cost of an online submission) for careless and clumsy errors at your end.

Hi,
As we would need access to your record to answer your query, you will need to contact our Self Assessment team for advice:
Self Assessment: general enquiries
Thank you.