Not sure it's even possible? Isn't the tank supposed to be gravity fed from a header tank above it....then again that might just fall into the 80% bollocks category!
It's possible if it's a newer, unvented system I think, as the water pressure isn't reliant on the header tank but comes from the mains. Not 100% sure on that though.
If it's an older, vented system then the cold water tank has to be higher than the Cylinder as you say.
Either way it's going to be a major job, if possible at all. I would expect it to come in well over a grand.