Thanks for all the suggestions - I have just moved the GPS sensor behind the dials.
Quite simple in the end, and I am now getting really strong pick-up of satellites. I haven't got any photos but:
Remove RNS in usual way to get access to the back of it, and the GPS.
Lower steering wheel to bottom, and pull out as near to the driver as poss.
There is a piece of plastic at the bottom of the instrument panel which attaches to the leatherette cover between the panel and the steering column. There is a spring clip on each side, so all you need to do is grab hold at each and and gently pull towards you/slightly up. It comes off quite easily.
There are two T20 screws under the dials - remove those, and gently pull the dials towards you. There is a multi-plug on the back, so no cables to worry about.
You can now pass the GPS through behind where the dials go - there is a metal bar at the top which the magnetic GPS will stick to - leaving a pretty thin layer of dash between the top of the GPS and the sky!
I secured the cable with some masking tape, and put back together.
Works well!