Sounds like you've bought a diesel that's borderline not fit for purpose if you live somewhere that you can't go for a decent drive without stopping. As it was a second hand sale though you're stuck with it. Give the nearest dealer a call and ask for their advice. In essence though it's time not constant speed that clears it, so slowing down a bit doesn't stop regeneration from completing, it just delays it. You're in Suffolk - go for a trundle along the A12 or the A14 then go home again and see what that does.
You've said it's a 2007 model, so over 5 years old and likely to be in range of needing a new DPF anyway. If the light stays on after this trundle around Suffolk have a mechanic look at it, particularly WRT to the sensor - they can clog up and give false readings; also the regeneration fluid which is in a small reservoir and is used to assist the burn process (and costs more than unicorn tears to buy). When mine went the first attempt at fixing it was to clean and refit the sensor and clear the error codes from the ECU but it soon resurfaced. Reality is that you might be in range of a bit of a bill.