There's a service that has been running in the south west for around three years where a man in a van turns up and can test your broadband service independently for a one-off fee. If the problem is with your line, the report will say so and you can then wave this in front of your ISP's nose. If there is a fault with your equipment setup, this is also identified.
Not sure how far this service reaches - I know it is in London and the south west, but not sure about other areas.
If you need more info, please let me know and I'll see if I can dig out the info.
SHARK!
For every positive action, there's an equal and opposite government plan.