Windscreen cleaner

I'd start with an alcohol based solvent, isopropyl alcohol or meths. If that doesn't work, the next stage is acetone, but that needs a lorra lorra caution as it's a very enthusiastic solvent.

Acetone might also dissolve, or at least seriously soften any old cellulose paint, but if the car is less than 30 years old its probably covered in "2 pack"

Nascar and Le Mans endurance racers allegedly clear their screens with Coca Cola, but that might just be an urban myth

Before I gave up going to car washes I used to ask them not to apply wax which is usually in the final rinse. Some did others not in the end I gave up and bought myself a pressure washer and did it myself. Since then I have never had a juddering windscreen wiper. As for removers of wax I found Auto Glym fast glass cleaner effective and not harmful to paintwork.

I use this stuff and a microfibre cloth, not cheap but it’s magic:

Only thing listed in the MSDS is 1-10% ethanol ...

washing up liquid and hard work

Well, it works a treat. The question is, what else does it contain that isn’t on the safety sheet, if anything?

I'd add a modicum of bleach to the washing up liquid. I use bleach + washing up liquid in my screen-wash bottle (just a squirt of each).

Given that it says it's colourless, practically odourless, relative density 0.99 and pH of 7, my guess would be water :-)

Bleach just turns grease into washing up liquid anyway. Unless you have serious grease issues its a tad overkill

Microfibre? ITYM polyester.

Well, it appears that we have the complete answer to Jim the Geordie’s problem:

A cheap mixture of methylated spirits and water, max ratio 10:90, and a cheap polyester cleaning cloth.