An update:
Good news, I have good oil pressure!
This was with a coolish engine but I never saw pressure like that on the Smith’s gauge. Just after this I reconnected the Smith’s gauge back up and guess what - 20psi.
So it’s either the gauge or the pipe. Has anyone had issues with the original Smith’s gauges please? Do they go out of calibration, can it be re-calibratedd or is it junk (assuming the pipe is okay)?
The XK120 uses a mechanical oil pressure gauge. Could the line be plugged?
Thanks guys. So it would appear that the XK120 gauge cannot be adjusted in the same way that the E type gauge can be.
I took the line out through the bulkhead and made sure that it was clear and then I took a plunge on a recon gauge - PROBLEM SOLVED!!
I haven’t had the car out as it’s rainy and horrible here in the UK but idle from cold I have a good 65psi which is maybe 20psi plus more than I was seeing on the old gauge.
Thanks for everyone’s input 
Only just looked at this thread, it does sound as though there have been a few short cuts taken, sludge and nuts, it sounds like you might be good to go when you get that day of sunshine.
