The engineer has been and gone. No fault found with the line (which I think we already knew/expected).
He's reset my line and my current stats are listed below, but it's likely they will drop again based on stability and max speed now available. The max speed I can now get is apparently approx 24-25mb download because of the increased amount of people connected to the cabinet / on the line. So basically, I'm not going to get back to where I was before at 28-30mb.
I think it may have been useful for this to have been mentioned as part of our conversation.
So I've paid for fibre on a 40mb circuit, but it's likely to only be available to offer the approx 20mb (once it settles again) because I wasn't told my speed would decrease naturally as more people/connections were added? Seems to feel like a bit like false advertising to me? Apparently if I paid for super fast fibre on an 80mb circuit, it's likely that I'd lose approx 50% speed again of that.
On the basis of the lack of information above, which may have explained the natural drop in my speed over time, I wouldn't be expecting any engineer charges. In the meantime I will monitor the current connection speed/stability following the line reset.
The issue of speeds dropping over time as more connections are made within the DSLAM cab is one that is known about, and is normally down to crosstalk interference generated by the high frequencies within the DSLAM cab. There is a fix that BTOR can use, called Vectoring, but it's not on a general roll-out across the infrastructure as yet.
If it's any consolation, when I first went live I was getting around 70Meg down, but some years (and a fuller DSLAM cab) later I'm down to 53Meg due to crosstalk.
It's something that ALL ISPs should make customers aware of when signing up for FTTC services IMHO.