Thanks a lot @hornbyben for the detailed writeup!
Can I ask how did you connect the pump to the boiler when you decided to remove and not use at all the (faulty) membrane regulator? Did you need extra connecting parts for that?
Also regarding laser cutting a cover as per the CAD file, it can be done online here for around £50 £35 + £6 delivery, for a 1.5mm thick 304 2B stainless steel.