Contract I work on is exclusively WB boilers. So work on nothing but. Like every boiler they have their foibles. Most of which can be eradicated by fitting onto clean system and fitting a mini expansion on cold mains ( left hand block and water turbine adaptor are prone to leaking otherwise)...
Try squirting a bit of washing up liquid in the pump with the nut removed. Can help free things up.
We used to have a fair few of these on a contact I worked on. if its never been serviced over the years just a heads up that the flues had a nasty habit of rotting worth checking/ getting...
. Unless the coils gone (unlikely) there shouldnt be much chance of any contamination its basically just copper. When cleaning header tank any crud/sediment will have been disturbed and will have to work through system couple of baths and it should be running clear again. Also make sure lids...
Can sometimes be better getting a fixed price repair from baxi once you take into account the price of a new pcb. Always worth having a look for scorch marks and checking harness connections