When I stayed near Newmarket in 2001/2 I used to see this about at various times. I always meant to try and get a picture of this handsome machine but never managed to get one.
It may be handsome, but it is not a very safe design.
The round tank is narrow and has a high center of gravity, both of which contribute to an increased tendency to roll over.
A full-width pentagonal or hexagonal section tank would be much safer, and numerous examples of this type of structure (sometimes also referred to as "T tanks") are to be found on this site.
In the US, more than 70% of apparatus accidents involve tankers, and these are mostly single-vehicle rollovers. High, narrow round tanks are of course the least expensive to make, but also the least safe...
Added by Chris Hall on 22 April 2013.