It is in fact a Leyland product, based on the Sherpa sheet metal but on a much beefier chassis to accommodate a bigger payload - marketed in the Netherlands by DAF and called a 400 model because it had a 4 tonne payload.
It was quite popular as a cheaper alternative to Ford Transits and the like, but not many were converted for fire service use.
Although the Sherpa officially lost Sherpa name in 1984 the name stuck with users till the end in 2006. Bigger versions like this were known as 'Fat Sherpas'. 400 referred approx to gross weight not load, it would have struggled with 4 tonne in the back!
Added by CraigW on 20 December 2017.
The GVW was actually 3.5 tonnes - the 400 was a bit of a marketing exaggeration. There were also 3.1 and 2.8 tonne versions.
Added by Barry on 12 January 2008.