I am not meaning to offend, just wondered if you have changed the plate heat exchanger on this particular boiler Baxi Duo Tec 40 HE, or is just general advice.
The manual which I have just downloaded does say to remove the gas valve, but after looking at the boiler it seem you can just remove two screws, after isolating and draining the boiler.
PS. I have already changed the expansion vessel so know how to make the boiler safe before attempting this.
As advised, no need to replace plate hex. Easy to clean and re-use. Get some Spirit Of Salts. Before using, try to flush out as much of the rust particles from the primary side of plate as possible. Then sit the hex in a glass dish outside and fill both the primary and hot water sides of the hex with the acid. Wear eye protection and avoid breathing the fumes. Leave in for 10 to 15 mins and carefully put hex in bucket of water. When the acid is drained out, flush the plate with a hose trying to back flush the primary side. Keep running water through it until no more debris comes out and water runs clear.
Sprits Of Salts will shift both the rust particles from primary side and the limescale from the hot water side if you are in a hard water area. When cleaned with the above method, the hex should work as good as new. If you have rust particles in the hex, best to fit a Magnaclean filter or similar to stop them building up again.