Neutral position sensor..
 

Anyone had to do one on an Blobeye six speed ?? Mine is putting the CEL on.

Yep, you need it on ramps and to drop the box a bit to get to it as it is right on top of the gearbox in the tunnel and you can't get a spanner/ socket on it

Any "how to" guides on replacing one? Had a search, can't find anything :(

Yea as above let the box hang down in the arse a bit and take bad one out, put new one in simples :)

Is it just a case of taking it off the mount to let it hang down? Is it easy enough to get into once it's dropped?

put a gear box stand under rear of it, take the bolts out of the cradel, and let it down a bit.
Its not a case of it being easier, it has to be dropped to get access.