Hi pcc and welcome to the forum
First of all your IP cannot move the goalposts! Your IVA is set in tablets of stone and is based upon the proposals you put to creditors at the meeting, with any modifications proposed by creditors and agreed by you.
I do not understand the hassle they are giving you - which firm are you using and why did you choose them to represent you? If your payments are being made regularly by standing order, they should use their own records to check up on you and not phone you directly. This is a sign of internal inefficiency.
With regard to increased mortgage payments, this is going to affect everyone who has a mortgage and is in an IVA. If you genuinely cannot absorb the increase, then you must discuss the possibility of a reduction in payments with your IP, who will be able to call a further meeting of your creditors to consider a revised offer.
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
For further details contact me at
http://www.melaniegiles.com and view my IVA blog at:
http://melaniegiles.blogs.iva.co.uk