That is very enterprising of you!

http://www.iherb.com/Kirkman-Labs-Ma...oz-113-g/38169
iherb.com ships to UK...but this is probably too pricey, compared to yours, but it has a special cream base to carry the magnesium thru the skin. 100mg/gram measure. Using 2 grams a day would last about 60 days.

You might ask your pharmacist locally if he has transdermal gels, available for you to purchase. You could mix it that way. Less fuss. Another base to try is Aquaphor...it incorporates liquids better than pet. jelly alone.
Instead of cooking it on the stove you can microwave it.
A short cut is to put all ingredients in a slightly larger jar (you can purchase an 8oz or 16 oz ointment jar at a pharmacy.... have your mag dissolved in some water a head of time. Put it in the wave and heat for 15 secs. Check it, and do 15sec more if needed until everything is melted. It doesn't have to be hot, just melted. Then put the lid on and shake vigorously for a minute --
the Aquaphor ingredients will suspend the liquid pretty well. Then put in the freezer for an hour or so. It will quickly gel with no mixing, if you did it right. Don't leave long in the wave...as it can catch fire (petroleum distillate)--hence 15 sec bursts to avoid this. . Do NOT wave it with the cover on. (just put some paper over it in case it spits. If it needs to, you can repeat this until you find the right time periods to use. This is how we make some topicals in the pharmacy.(large orders with heat stable ingredients.) This uses little equipment, and gives a nice finished lotion in the end. Most pharmacies will sell you the jars you need. (In order to shake this hot stuff up you need a good jar like the pharmacy jars. Reused food jars may leak.) Making it in the jar is handy because it stays in the jar and doesn't need to be moved at all.

Much of the weight of the epsom salts is water already as the crystals have water attached. I see Aquaphor is available now in UK too... you can try to get it by the pound, as that is most affordable. Here, WalMart sells it. My husband uses it on his psoriasis.