The lever should be in line with the pipe and valve body, not across it. There's often a yellow sticker with an on-off diagram near the meter.
I've had callouts where something has fallen against the lever and knocked it. Also I had one where the lever was only partially closed and the cooker worked but not the boiler, which is why I suggested you check.
If it's not that, I can't think of anything else you could try yourself - it's probably Yellow Pages time.