I too have the powder from Maplins.
I mixed half my powder into half the amount of warm water. From memory I think I used about 50ml water.
I keep mine in an old medicine bottle (washed thoroughly...), and simply pour our as much as is needed into a small plastic tub (from an Indian Takeaway, would you believe).
I keep *this* tub warm during etching by floating it on a larger tub of hot water, but have also been known to put the takeaway tub on a USB cup-warmer - which also does the job.
Good luck with your etching, I'm sure you'll get the hang of it.
p.s. Warming your boards before immersion can also help. I leave mine on a radiator (flat, face down) for 10 minutes or so while I prepare the etching bath.