No reason why you shouldn't call the original installer, but if you're expecting anything to be done for free you're going to be disappointed. Are you sure the leak is actually a leak and not just some other residue? Your photograph of the flue is cr@p so I can't comment on that, because it's too dark to see anything clearly. As for the boiler, get a price from your local chap and from Baxi to repair. Might be cheaper to get Baxi to do a fixed-price repair on it.
Thanks so much for the advice . The BG guy says it's a leak but the water doesn't actually drop onto the floor. Is it alright to get the boiler fixed first and get hot water running then deal with the flue brackets?
When you call out an RGI to a boiler fault (BG or anyone else), he/she will be taking responsibility for the appliance and not just rectifying the immediate cause for it to no longer "work". It is this transfer of 'ownership' which strives to ensure continued safe operation and use of such an appliance. It is also why end users should not be shocked when they are presented with costs for additional work, beyond the nature of their original reason to call upon the services of an engineer!
