Mike
Ah! Where was the fuel tank on the real CMP? That us what I wondered too on examining OD's CMP model, as it does not have an obvious fuel tank. From the pictures wamwig has posted, and others, the fuel tank is below and behind the cab (I don't know if there is one tank, or two, one on each side).
On some CMP varieties, the tank is obvious; on other versions - as modelled by OD - it is much less obvious. Oxford Diecast seem to have modelled part of the tank. I gather the fuel capacity was 24 gallons, which at 7 mpg does not make a long range!
John