is the 20k units on top of the estimated reading your supplier has racked up in the mean time or simply 20k more than your last real reading.
If the later, then you probably don't owe that much. If you had a reading at 10k however many years ago and it's 30k now then you've been paying based on an estimate and it's unlikely to be insanely far off unless you changed your usage pattern significantly or moved in near to the first reading.
If you have to work in a conservatory, get new improved double glazing put in and make sure the floor is insulated. Conservatories will be colder but they don't have to be freezing or be incredibly expensive to fix.
I moved into a new place with a extension at the back with all crappy old double glazing, some blown but very thin. It gets significantly colder in there than anywhere else. It costs something like £40 per panel for, I forget the size, around 100x50cm panels. To replace the glazing all around would cost me around £400 + say £50 in some beading/caulking depending on how I do it, or maybe a grand or so if I have someone do it. Floor is probably the biggest heatloss, still need to get someone in to have a closer look but it appears to be not well if at all insulated, flooring on top of concrete so is cold in winter. Something the previous owners did that I would also have done is replace the glass roof with a normal roof which increases insulation massively and also gets rid of how stupidly loud conservatories can be in the rain.
If you're burning truly silly cash on heating then it will usually end up better to spend it upfront on making the place more insulated and it will also be consistently more comfortable to work in.
Another note, if you are doing business work at home you can claim tax relief against spending on extra heating usage for a home office, that kind of thing.