On new years day i took my car out. There was intermittent rain for the early part of the drive. The wipers worked fine.

On the way back i had fog/mist which wet the screen but no wipers. I stopped in the services and checked the nut on the motor connection, this was tight. However there was no noise of power at the motor.

This morning i've gone out to check for faults and the wipers worked fine. No blown fuses, all electrics work fine and there were no loose wires.

Any ideas? The linkage jas been stripped and lubed previously.

Well that could be one of a lot of things unfortunately. I assume all speeds did not work. First suspect is a poor fuse connection. Clean the contacts and replace the fuse (or clean the old one). The motor is suspect but that is more difficult to check and replace. If it fails again, I would tap it hard with something and see if it starts again. The wiper switch is still suspect. Lastly there are 2 screws that hold the switches in place. Remove the horn and make sure all 4 corner screws are tight. The ground connection is made here. I think this may cause the opposite problem (occasionally wiping when it shouldn't) but this is easy to check just the same.

Often I spin the fuses in the holder and wiper (etc) will start working. I haven't run into this issue with my 914, but it's common on my '67 bug which sees more DD through the season.

Tech tip if you remove the fuse block and soak it in vinegar the connector/holders come out shiny brass. I have two fuse blocks for my bug so I can swap them out fast without having to remember (label, etc) the order.

+ 1 on checking the 4 screws under steering wheel. Upper right corner is the wiper switch ground up to 75

Yes, the trick is to have the spare fuse panel and move each wire individually. Takes 10 minutes. Ebay sells a pack of new fuses for $10. Put the old ones in a baggie in your glove compartment so you can help out teeners in need.

I've found replacement glass for the mirror but it says it is 69 to 72. However the mirrors i see for sale are described as 70 to 76.

Is there a difference between mirrors from factory initially? If so can i buy and make fit the early glass?

true early mirrors are a bit smaller.
they also have more of a sharp, right-angle bend on the arm - the late mirrors have more of a gentle curve.

JFG,
Check J-13 relay on the fuse panel under the dash. This is labeled in the prints as the fan relay, but should have been labeled a power relay. It supplies power to the top of fuse #8, I think. Fuse 8 feed the wipers, horn, fresh air blower (fan).
If everything is working well now, the next time the wipers don't work, try blowing the horn. No horn, reach under the dash and wiggle the relays and see if the symptoms change.
If you find that J-13 is in fact causing the issues of intermittent power, remove it and clean the relay prongs and with the relay turned upside down looking at the prongs, slightly spread all of the prongs very slightly. They look similar to a pie divided into 4 pieces if I remember correctly. When you put the relay back in, it may feel a little tighter and the intermittent power issue should be fixed.
Tom

I thought horn relay supplied power to fuse 8

Nope, horn relay (J4) is powered from top of fuse #8. Fresh air fan relay (J13) puts power to the same electrical point on fuse #8. The fresh air fan relay should have been called a power relay. Per the print below.
Tom