Hi Chris,
I live in North Bristol and at a recent local community meeting I asked this very question to the local Neighbourhood Team Inspector.
His response was that it was illegal but that they had to use discretion. So, if you were a 10 year old riding slowly, unlikely to cause an incident, they'd probably let it slide. However, he cited a case of a 'fully grown adult' going 'at top speed' down a pavement on a local highstreet packed with people - who would of course cause a lot of injury if they hit someone. They caught up with him, put him through the courts, and he ended up with a £500 fine.
Hope that helps,
MG