With the covering letter, I think I would email it to GT as well just to make sure they receive it.
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley. http://kallis3.blogs.iva.co.uk
This seems to have been a modification by HMRC at the time of the IVA so may be difficult to overcome. However it may be worth speaking to Shaun Vickery from Select who posts here and see if you can raise a lump sum against the house. This may be acceptable to HMRC.
If this is true, you would have had to agree to it at the time so it shouldn't have come as a surprise. Do you remember agreeing to anything like this at the start of your IVA? You need to check your paperwork carefully.