Took her out for a 20 mile run this morning and there was no trace of any petrol smell (and got caught in a snowstorm!). And adding to that the advice on the RHOCaR forum and the fact that there was a very strong smell of petrol in the garage a couple of hours after returning yesterday I think I have narrowed down the problem. This is the 'wiring diagram' of the fuel system
The previous owner tells me he replaced the braided pipe between the filter and injection rail as it was being corroded by modern, ethanol added, fuel. So I suspect the other piece of braided pipe is going the same way. Under normal driving conditions there is no problem, but if the car remains stationary for more than a few days then the pipe starts to absorb petrol. Once the car is started nothing happens for the first couple of minutes but when the engine heats up it starts to evaporate the absorbed petrol (and the smell comes up through the dashboard and tunnel). However, once the car is put away with the engine still hot it continues to evaporate for a few hours, hence the smell in the garage a couple of hours later. By the next morning all the absorbed petrol has evaporated so the smell goes away.
So obviously the fix is to replace the second piece of braided pipe with the latest ethanol-resisted fuel pipe. But this is not an easy fix so I will live with for now, knowing that a quick 'blast around the block' after a lay up will cure the problem overnight.