By "£18,244 remaining", I take it you mean that this is the total of your remaining payments ? If so, does this include 12 extra months worth for equity? Are you, in fact, a homeowner ?
How long have you got left in the IVA ?
Reductions in the F&F lump sum offered are common and some firms are more open to this than others, but, at the end of the day, it is your IVA, not theirs. Usually the basis of a reduction, apart from the bottom line of affordability, is that you reflect, in part, the savings creditors are potentially making in both time and admin costs by getting a lump sum early.
If you are knocking a couple of years off, I would say that the £16,500 has legs.
Bear in mind, also, that, with the ever increasing cost of living crisis, there will be casualties in the IVA world. I imagine that a number of existing arrangements will fail, so, from the creditors point of view, getting a lump now, rather than risk getting nothing in the near future, has to be inviting --- "Bird in the hand" and all that.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014