You could possibly remove the plate heat exchanger without removing the gas valve, if you have at the very least a quality pair of grips (water pump pliers) like Bahco.
You will need a few 1/2" and a few 3/4" fibre washers and a particular red o ring. You can get them all in a kit of washers but it is not a cheap kit, something like £30 with the VAT and there is a load of stuff in there you don't need, that is worth having for a heating engineer, all the seals for the whole boiler.
Isolate flow and return, isolate domestic water inlet, drain boiler.
Anyhow to get it off, which is not easy at all, undo the three nuts a little at a time each, the fourth connection has the red o ring and has to be forced up and off once the nuts are undone. It takes what one of us would consider a typical amount of force to get it free, a novice would quite likely give up scared of breaking something (which is a real possibility, at least bending something which would then require similar force to get back into place for the replacement of plate hx later). If this part of the operation goes smoothly (unlikely for a first timer) the seal on the other end of the pipe will remain sealed. Too much wrything and those also have to be seperated and new seals applied (supplied in kit). Worst case scenario to manage the job you might have to remove the pump, always nice to be able to remove gas valve for access but not essential. Can be done with pump and gas valve in situ.
Once off if you want change it, no idea of new cost sorry. No cheap ones available for this boiler, or at least I haven't spotted one yet. If anyone can source them please let me know.
It can be cleaned, it has four hydraulic connections in it, the larger two are primary, full of black stuff, the staining will give them away. These are the ones to clean. You can get a pretty good result just blasting mains pressure cold water through it both ways, agitating it with the palm of your hand with hammer bolws. No need to bother with the other side. In an emergency (late evening far away from home, didn't have one with me, I have poured a bottle of malt vinegar (borrowed from the students just happy to get some hot water) and let that soak in, then save it in a bowl, repeat the mains pressure flush, go back to vinegar, etc. After reassembly the boiler functioned like new for hot water.
Reassembly is quite difficult, the pipes don't want to approach the unions at the right angle, you need plumbers hands, extremely strong and dextrous. Most plumbing jobs are given us because of week hands and lack of tenacity of approach. To do tasks like get this plate heat exchanger back on you have to be as tenacious as a pit bull holding on to it's leed suspended 6ft in the air. But it is possible, I have never failed, I am not allowed to.
Afterwards, you can't really powerflush to be fair, but you could follow Chrises advise on mains flushing. Also treat water over four weeks with sentinel x400.
magnetic filter such as magnaclean is excellent buy I'd have one at home if I had a plate heat exchanger and a low water content boiler,
