Finally got an oil pressure reading, but it did cost me an oil filter...
Tonight's mission was to check the oil pressure:
Battery connected - check
Starter motor wiring okay - check.
Drive belt clear of obstructions - check.
Oil filled(!) - check
Spark plugs removed - check
Oil pressure gauge temporarily tee-ed in to oil pressure switch - check
Time to turn the key. Click, whirrr. All good. More whirrring, no oil pressure. Scratching of head. Key turned again. Click, whirrr. Still no oil pressure. Hmmmm.
Had a quick search on Locostbuilders. Seems to be a not uncommon problem. One solution is to prime the pump and the oil filter. Tried this by pumping some oil down the oil filter threaded spindle. Also half filled the oil filter. Tried cranking again. Still no joy.
Frustration building. Then thought, let's just prime the pump via the oil filter inlet and crank over, with no oil filter fitted. At this point past caring if the garage walls get sprayed with oil.
Tried cranking again. Success! Maybe it worked better when there was no back pressure on the pump? Anyway, refitted the oil filter and cranked for longer. Oil pressure built up steadily to around 40 to 50 PSI. Job done.
Next job is to find a replacement oil filter. In my haste I trashed the casing of the old one with my cheap chain type oil filter wrench.