http://www.city-sightseeing.com/
I'm thinking about filling in my trip with quite a few of these bus tours when i don't have a whole lot of time in a particular city, as they seem fairly good value and also reasonably flexible considering you can get on and off at any point, and it also looks like a good way to ensure you get a chance to see a lot of the highlights in each city in a short period of time.
Has anybody used any of them before, any in particular they would or wouldn't recommend?
The city's I'm thinking about doing the bus tours in are,
Cardiff
Glasgow
Edinburgh
Newcastle
Liverpool
Luxembourg
Brussels
Porto
Lisbon
Barcelona