hmmm, here in the US you can order a 7 pound tub for 13 dollars US. from the online chemistry sites.
cold water should be just fine. sounds like the concentration of acid isn't high enough if rust isn't disappearing.
you should see rust results in as little as a few hours.
I just swirl the mixture a couple of times a day.
One time I left some spokes in the solution for a few weeks. didn't hurt them but a green residue formed that had to be wiped off.