Hi, I’m sorry hear you’re having boiler trouble. This is quite common at the moment and there’s a lot of grey area between safety checks on your appliance and a “proper” service. It’s a bit of a minefield for clients but it would be handy when booking to find out exactly what the service entails. The problem you’re experiencing sounds like a faulty expansion vessel, we cover the recharge of this under our annual boiler service, but some businesses don’t and unless the manufacturers instructions specifically state this needs to be checked, they’re not under any requirements to do so - for us, it’s an added value benefit for the client where we try our best ti alleviate any issues in the near future. There is also a possibility that the fault has arisen since the service and they may have checked it at the time and it was ok - only they will be able to tell you that. The radiator resiting shouldn’t have any affect on the overflow situation - if it was leaking the pressure would drop but it wouldn’t be coming out the overflow.
In terms of the repair being carried out a year ago, expansion vessels in the first instance would usually be recharged to factory pressures before opting to replace unless it was clearly split so it wouldn’t be uncommon for this situation to arise again as it’s very common. (That’s if it was the vessel they sorted previously). As long as they’ve serviced to the manufacturers instructions, they’ve done nothing wrong, anything additional is just an addition provided by companies for whatever they see fit. Hope this helps a bit!