You are perfectly entitled to get your own valuations to challenge theirs. I would also ask them for a detailed breakdown of their own calculation.
An online valuation is, at best, a guess and is, as you say, not based on YOUR property (which hasn't been maintained for 5 years) and is based on sales of properties over a large geographical area which have all been staged for resale.
I am not aware of any obligation for you to use their named valuer. Forced into using a nominated firm calls into question the independence of such a valuation. In most cases IPs would not rely on a single valuation, but would require you to present 2 or 3 for comparison - and would probably accept a mid line figure ---- so, 'What is good for the goose .....'
I do not know what version IVA terms yours was drafted under, but Annex 6 to the Standard terms under the 2016 Protocol stae: " In month x of my arrangement, (normally 6 months from the end of the arrangement) an open market valuation will be carried out on the property by an independent professional valuer. ....."
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014