any cheap device to measure average speed of car just like police speed gun

[dinoy]

The police may be interested if the current speed limit is 30mph and buses are routinely ignoring it, but otherwise I doubt it. The only way you'll find out is to make a complaint to the police about speeding buses (if they are indeed speeding) and ask them to take enforcement action.

As grober said, you need to get a local councillor on your side to take it up as an issue.

they are all involved we had council meeting yesterday which was the usual white wash we will do 20 mph zone.

but they did put a not to contact tfl and ask them to ensure buses do normal not go over regularly