if you can afford petrol, there is no reason to buy diesel.
regardin diesel engine - there is ony one common problem - egr valve. This is the cause of high consumption and/or going in the limp mode, loss of power etc.
I don't know whether you have ****ty fuel in GB, or it is because of the driving style (you are pushed to drive too slowly) - but keeping the engine on 1600rpm on 6th doesn't help the situation either.
Anyway, egr needs to be clean, and thats it. Manual how to clean is here, it is not so difficult to do this at home.
Other question is, why only early 2.0tdi have such a problem (they had problem not with egr, but with the cat) and other marques not. I don't know. But many IS owner have new egr valve, and less and less people are coming here with complaints, so I think that it has been solved.