As with the previous 2 posters, I'd suggest keeping the 20quid/hour
increment right through - and specifying a mileage or time range for
the "included travel". Some people may not be calling because they
think they're out of range - or vice versa.

You could also, on the same charging basis, offer a light removals
service (e.g. just move some boxes from A to B - or move the washing
machine to auntie's house and replumb at new location). That might be
an opportunity for some potential new customers to meet you and gain
confidence in you.

One other thing to think about is how you increase prices when it
becomes necessary. A uniformly incremental structure means you can
just add 5 quid at every price point at sometime in the future.

If it's any consolation, you're going through the classic new own-
business/self-employed price adjustment. I heard all the warnings
about "you'll set your prices too low to start with" - and still did
it myself.