Engineer turned up. Switched boiler back on.
Ran perfectly. No gurgling or anything.
He thought the fan sounded a tad noisy (sounded normal to me). It mixes the air and gas before combustion. He's coming back today to fit a new one. Can't imagine why that would cause gurgling/boiling noises but hey ho.
I wonder if my furtling with the condensate soakaway broke an air lock or something. Can't say I noticed a rush of condensate when I was investigating.
Maybe it was just pining for a bit of attention.
All v odd.
