I use this, £23 for 25m roll in 1.5mm
http://www.toolstation.com/shop/Electrical/d190/Cable/sd2730/3+Core+Flex+Rubber+Cable+(3183TRS)/p80794
Works out the same price wise for 25m when you add the delivery but you can go and get this locally at 19:00 or on a Sunday
Grab yourself a couple of plugs and some double rubber sockets and make a couple of extension leads with the rest
http://www.toolstation.com/shop/Electrical/d190/Cable/sd2730/3+Core+Flex+Rubber+Cable+(3183TRS)/p80794
Works out the same price wise for 25m when you add the delivery but you can go and get this locally at 19:00 or on a Sunday
Grab yourself a couple of plugs and some double rubber sockets and make a couple of extension leads with the rest