Baxi Gas Fire Low Heat Output

[BobTheRocker]

I have a Baxi Bermuda back boiler with an SP3 firefront. Over the last couple of years the heat output (and flame heat/size) has gradually decreased to such a point that on setting 4 (full) the heat output and flame size is of the same size it was on 1 when it was brand new. Any ideas on what it might be. I'm thinking maybe the gas valve/tap(FFD) is at fault. I see on the parts list it also has an injector and electro magnetic unit, could either of these be at fault. Has anyone had any experience of this fault on this model or similar. Really wanted some info before I go paying a plumber silly money to replace a part that possibly does not need replacing.