Just adding something that I have come across with PE pipe, if a screw is put into the pipe at installation accidentally when putting floors back down , screwing plasterboard etc, sometimes it seals and then through time the screw rusts and starts leaking a few months down the line, not saying that is your problem but is worth bearing in mind.
Cannot understand why FOC comes to mind before any attemp is made to identify the issue. Would the poster pay double charge for calling out the installer if it was not his woman ship to blame?
Above is very similar to "it is the same problem as the last time" I hear time and again when boilers repaired previously break down again
Indeed hence my comment on jumping in feet first. Had one recently "you only serviced the boiler recently why has it broken?".
Well madam, for a start it was 8 months ago, and, the fan has failed. Nothing to do with servicing on a 7 year old boiler.