Trouble is.... I don't think that's viable in the eyes of COP due to ground clearance... On top of that it would have to be twin tiny tanks to fit the gap and probably no bigger in reality than a full torroidal in the wheel well!!
Hmm. :-/ I must admit I've wondered about doing similar things. Ground clearance may be an issue, as said. I guess with the petrol tank being 75 litres volume it would yield space for a 60 litre LPG doughnut tank or thereabouts. No better than a spare wheel tank.
However, IIRC, the estate petrol tank is a pretty irregular shape that fits around the spare wheel well, so, unlike the saloon one, its' removal might not yield useable space.
The other option would be to remove the spare wheel well completely and weld in a flat panel, then remove the petrol tank and make what you can of the remaining space - or replace the wheel well with a much larger "sump" into which you could fit a larger LPG tank thus avoiding issues of ground clearance and exhaust proximity.
I would also want to be 100% sure that LPG is the fuel of choice in the long term and unlikely to be milked by the government before doing anything this permanent to a good car.
Kevin