Heater Core Flush

Realised on a long journey "farn sarf" that my climate control doesn't provide much heat, it's lukewarm at best. After ignoring the missus going on about "there's cold air hitting me again" for a few weeks, I finally realised she was right....but don't tell her.

I want to try flushing the heater core but I have two questions:

1) Halfords sell a VW 5ltr bottle of coolant, how much coolant is required from empty? Do I need that much or can I get some VW stuff off ebay in 1.5ltr bottles?

2) Where is the easiest place to get the required hose. It looks like I need 20mm/21mm or 22mm diameter hose but clearly don't want to buy an entire reel for 50 quid?

Cheers mate. Managed to get some pipe so will try this shortly.

After borrowing some tools from B5Nut, got the top pipe off (took half an hour!), got the battery out (which nearly broke my back) but couldn't remove the lower pipe. Tried for half an hour but got worried I was going to split the hose. Put it back together and will go forward with B5's suggestion to remove the lower radiator hose and flush the whole system. At least in that case I will be removing all the crud.

Update: Wasn't straightforward, had to remove the thermostat and one of the torx is in a crap place. Managed to flush the system out properly, refilled (took about 2L G12++ and 2L water thereabouts). Heater works wonderfully now!