tunnels for an underground spitfire factory on the Wargrave road just outside Henley

You're on my old turf now. I thought you were referring to Warren Row, first a chalk mine, then extended and used for making aircraft parts, then during the cold war in the 1950s designated a Regional Seat of Government (RSG) and now a glorified wine cellar, but now remember there was also an underground factory on the Wargrave Road.

Yep, as you come out of Henley, just as the road starts to rise from being level next to the Thames.
Now houses data/file storage I believe.

In reply to a post by Zarjaz:

Nope, they still use the BT66.

What they don�t use, and haven�t for many years, is the brass screw terminals inside.
As you correctly say, these should/would be replaced with gel crimps.

Thanks for your reply, do appreciate your help.