Discovery LPG tank leak problem

Learned friends,

I recently bought a 1998 Disco V8i fitted with twin LPG tanks under
the vehicle, replacing the original fuel tank. I have noticed after
not using the vehicle for several weeks my LPG tanks are now nearly
empty, so I guess I have a leak somewhere!

The Disco has an LPG certificate dated 2001 which states that it is
fitted with 2x 40 litre tanks (water capacity). It also has a 30 litre
reserve type fuel tank.

With no power to the LPG system the tanks should be closed by the
spring on the valves - right? So how come I can loose gas from both
tanks? Does only one tank have a valve maybe?

I have lifted the fuel pump access plate in the back of the truck and
looked down, but am much not much wiser - looks cramped, so I guess
the whole cradle will need to be dropped down.

Does anyone have an idea of how hard this is going to be to fix?  I
have lots of experience with Land Rovers, but not much with LPG.

Should I get this looked at by a specialist? If so anybody recommend
an honest company in the Bournemouth area?

Also what parts would I need to order if I decide to do this repair
myself and does anyone have a diagram of how a twin tank system should
be fitted.

Any ideas / suggestions gratefully received.

Best regards to all,

Macman

p.s. Also posted this on uk.rec.cars.fuel.lpg