Coir

Thanks Veggie.

Spending a dull drizzly afternoon trawling the Web has come up with the cheapest at 75L for £11.20 free p&p
https://www.ebay.co.uk/itm/385531643011
I'll need some perlite too so will have to cost it up to see if it might be better buying in peat free compost (if i go peat free), but that won't be half as much fun.
20L perlite free p&p for £13 on Ebay.

I got 100L of perlite from eBay £23.50 delivered last week. Smaller bags are always way overpriced.