Odd Doon.... I will be really interested to learn what the issue turns out to be.

I'll get to the bottom of it one day,  just very limited opportunities and space to work on it

Fitted a cheap mechanical pressure guage instead of the warning light and it mirrors my electric guage albeit with a delay probably due to air in the pipe, so only bit of good news is my guage and sender are OK.

Took rocker covers off and there's a dribble of oil reaching the rockers so some is getting through but pressure on guage all over the place.

Looks like it's engine out.

Doon......   Having used the rocker cover off check many times now, and having seen a pro re-builder do the same, the flow should be far more than a dribble. I would take the engine out and strip it down. There is a good chance of one or more exhaust valves sticking open if the cylinder head/heads overheat.
My guess is there is some crap in the oilways which can also be in the pushrod tube but I have never seen all tubes blocked. It`s more usual in just one or two.

Re- capillary tube to an oil pressure gauge....  I have lost count of how many classic/vintage cars I have seen where these tubes, when clear material is used appear to have air in them,   

Don`t forget to pressure up the  oil cooler with an airline just to check it hasn`t collapsed within. I don`t think it`s coincidence that the issue followed an oil change.

[Take a look at photos.]

Bit of old school stuff from me - an ancient old schooler with no engineering training just decades of direct hands on experience.  (For you on the forum knew to engine mechanicals)

Take a look at photos. 54 year old engine. 54 year old capillary tube and 54 year old Smiths oil pressure gauge...   Don`t overthink things. (easy to say I know)....
The photo of the dash gauges is at start up from cold hence engine at "fast idle" due to choke cable positioning the 2 Stromberg carbs at fast idle position. (1500 rpm exactly where it should be). The thing to note is a stable oil pressure indication... not the actual oil pressure. Then take a look at the other photo showing the capillary tube itself. The dark green sections of tubing is the oil the other is air. You could try and disconnect and get air out but it wouldn`t take long for it to re-appear.

The important facts to gleen from such a scenario is a stable positive oil pressure. As an engine heats up, and especially after a long hot run, this pressure will very probably drop away in most cases but return when back to idling and cooling in effect. That`s all you need to know. The actual gauge reading is nominal at best unless you have a very expensive set up.

Dont worry Paul,,  I don't really pay much attention to the numbers on guages just to change and stability - except with Speedo and Rev counter

Hope you get sorted soon. I for one really do appreciate how restrictive space can be. I do everything from my buggy build to all other stuff in a tiny brick garage and minimalistic shed. Having one arm almost completely out of action since January has been a real eye opener. I am right handed and what a thing it is to realise how much your left arm/hand does in conjunction with the right. I am in awe of folks managing with one arm or one hand.   

Ancillaries off and oil out, engine out next week.
Will pressure wash it before dismantle after blocking all the holes then the work starts.
Never dismantled a Beetle engine so Haynes manual at the ready.

Dont have access to an airline,  have bike pumps that can store 200psi and let it go in one burst. Will try that with an old car valvev to get a seal on the oilways.
Did think of using the pressure washer on oil ways then drying the case in the oven- don't tell Lynn. Any thoughts?

Washing the case will be fine the water will evaporate.

Washing the case will be fine the water will evaporate.

Cheers Keith'
That's what I was thinking

You will enjoy dismantling and re-assembling the engine..... 

Made progress
Was going to add a pic of engine on stand but won't add images for some reason.

Will start major surgery tomorrow.
Fan housing and alternator off then pressure wash the dirt off after a soak in Gunk

Use a wax based sealant sparingly. Blue Hylomar or WellSeal.  Not silicone under any circumstance. (Wax based sealants ensure any excess within case can dissolve and not cause a blockage).

Plenty of engine oil on moving parts during re-assembly and of course the utmost cleanliness during re-assemble.

Absolutely Paul.

Started dismantling and bottled out before splitting the case so going to Farmer Rob so a grown up can do it.

I haven't really got the space or a clean enough work area.
A bit off putting when the Haynes manual says how different and complex it is.

Done old non crossbow Firds before and they're a piece of cake mounted on an engine stand,  can't do that if you have to split the case.

Wow..   100% baffled.... How can anyone like Haynes state the engine is complex. It could not be more basic.