I think Jason is correct if a payment is made wether it is money or payment in kind it would be deemed a professionally made boiler and as such would require CE marking (depending on Bar / ltr capacity)

Also the person holding the certification would need to produce his insurance details to say that they accept the liability on his behalf.

You can hold certification without being insured. (it's the insurers that want proof of capability )

One way you could look at it if a fellow club member holds the certification and he welds your boiler for no payment what so ever and the club inspector accepts this then the boiler should be ok for the club insurance.

As for coding he will need to hold the appropriate code say EN287 or ASME 9 for the correct material , cover the diameter range, the joint type, correct welding process , and correct welding position.

Your other option would be to do some test welds in front of your club boiler inspector get them tested then weld your own boiler. ( proof of being a competent welder)

Paul