Tax Return Attachment File Size

I have used Iris to generate an amended electronic partnership self assessment return for the 2010/11 tax year.  I consider it necessary to attach a PDF with a file size of 3.5Mb, which is clearly within HMRC's 5Mb size limit. 

Having tried and failed twice to submit this return I am told by Iris that in practice it is not possible to transmit files exceeding 1.5/2.0Mb because the internet connection drops out.  They are saying this issue is with HMRC, not my own ISP.

Has anyone come across this issue before?

Thanks

Tim