Could boiler heat exchanger be clogged?

My boiler seems to take ages to deliver hot water to the taps... over a minute, more to bathroom which is located upstairs...
The Boiler is located approx 4-5 meters from the kitchen tap, although it's located in the garage.

Also, my gas bills (it's a gas combi boiler, Potterton Combi 80, so it's more than a few years old) seem to be disproportionate to other peoples who have to heat larger homes...

Could the heat exchanger be clogged or something?

Hot water pressure is fine, although I've not actaully measured it...apart from this issue, the boiler works fine, and dleivers a good flow of hot water to the taps/shower and CH system.