Sorry but I have to say that �1,500 doesn't sound a lot at all. Most plumbers charge about �40 per hour and if the garden had to be dug up then this may have involved the hiring of equipment such as a compressor/road drill (for instance, if the leak was under a concrete drive) or the hire of a mini digger. It depends if the leak had to be found (i.e. the whole length of the garden had to be dug up) or was it obvious where it was?
This is why the water companies bombard us with 'buy our insurance and it'll save you money' leaflets. They may be annoying and unwanted, but I've never met someone who has had to claim that thinks they are a bad idea!!