Asda had a good deal on their own screenwash, 2.5L for £1. Mixed 1:1. Cheap as chips and does the job fine.
I bought 500ml of Holts stuff from a Shell service station this week for £3.50, mixed 1:1 and lasted no time. Wasn't all that great either.

Needless to say we use Renault branded stuff undiluted, but its pretty much the same as all the others and has worked so far down to -5 or -6, which is as low as its got here so far.
Doesn't that eat into your profit margins though? Surely it's worth about £50/litre when used to top up customers' bottles during services?
Nah, not really, it retails at 79p a litre, so compared to some 'branded' stuff, its actually quite cheap, albeit still double what supermarkets are charging - and we get de-icer from a third party very cheap too, so basically, because we are nice like that, the staff rarely pay for de-icer unless they are off on holiday when they need it. 