I really think you're better off to move in and see what happens but Power Flushing is really terribly straightforward - and there is absolutely no need to pay the £650 +VAT that BG want for doing it.
I hired a Power flush pump and chemicals(bought those!!) for the grand sum of £95 and followed the very detailed instructions to the letter. Took me 3 hours and it has given my CH and HW a new lease fo life. BG had told me that the boiler was irrepairable and could not stop it "kettling". Now it's a smooth and quiet as new and although it wont last forever (its 18 years old) I have bought time to work out what to do when im ready to replace it. I hired my unit from Kamco in St Albans, herts - sure there is soemwhere near you that can do the same