I have one that looks a lot like Steve Knightly's, but mine was made by Rob Armstrong in Coventry. Rob is always worth a look. Incidentally; as I remember, Steve doesn't tune his as a mandocello. Tunings (I think) are GDAE, or octave mandolin tuning, and sometimes GDAD, often used as an 'open' bouzouki tuning, although without the octave strings on the bass courses. To tune as a true mandocello, you need to be a full fifth lower, tuned CGDA. This requires a very heavy bass string, somewhere around .065 or so. Otherwise, the bass string is too 'floppy' on this scale length.

An alternative is to buy an octave mandolin. Fylde do a nice one, and there are lots of smaller builders around who will do bespoke instruments. At the less expensive end, I played one of these recently, and it was well made and sounded great. May be tricky to source in the UK though.
Good luck!
Tim