The sender is currently near the washer bottle and charcoal filter area.
Strange of Carlton Hydraulics to be like that, there mobile fitter I spoke to when we serviced his van was very helpful, and was positive they'd have something in the
shop that would fit it. As I even drew diagrams etc of what I meant. Maybe worth calling in with the sender and showing them what you want. Took me some explaining to the fitter.
Some people have bodged their 3/8" NPT senders into the M10x1 hole in the oil filter housing, with thread lock and sealer. Was never something that I wanted to do though as I wasn't keen on the idea.
Another option I thought about was using a machining
shop in Worksop where I had a couple of spigot rings made up for the wheels for £10. Not sure if they'd have been able to do it as never got round to nipping down and asking them.