I was going to ask the question of where can I get potting compost and growbags from as everything is shut but I can answer it myself now.
B&Q shops are running a click & collect service. You buy online, when your order is ready they email you and arrange a collection time, then you drive into their car-park and a member of staff will bring your order out to you.
Order before 4pm and you can collect after 1pm the next day. The shops are 'open' 8am-5pm.
Thought I'd share this with anyone who is planning on any gardening or d-i-y etc.
Good shout. Think most supermarkets are selling compost too? My Morrisons is anyway.