Most cars lose the fuel out of the float chamber in a week or so these days. It's just so volatile that it evaporates easily. As I've said before, modern petrol is blended purely with injection in mind.
Although in your case are you saying that the petrol is leaking into the manifold/engine? As you say it's not cheap, but I'd be surprised if your carb wasn't repairable?
Thanks for your post Neil. The funny thing is that the carb, pump, filter king (in fact everything in the engine bay) is impressively clean thanks to Tim Bagg the previous custodian. I don't know. I guess I'll sort the carb out first and go from there ....