Hi Iain,
How much lime did you take? Was it in the original container?
I weighed out 15kg of lime + a bit spare into the normal 20kg tub (poured out a few kg from a new tub into another one I had handy), enough for one fill per day for 6 days. (you can never get the last bit out of the tub I find)

I put the tub into a soft bag (just a small, light overnight bag I had that was roughly the right size), put a copy of the 'safe for travel' declaration from the manufacturer in the bag with it and checked it in as normal, maybe I was lucky but I had no hassles with customs.

By my reckonign it cost me £85 (£60 worth of lime and £25carriage), as opposed to £150 to buy it in Egypt from the boat. Also the lime was already bought and paid for so it 'felt' even less..

I now have a collection of 5litre tesco 'mineral' water bottles - which take 4kg of lime each, might use these in future as a) lighter and b) smaller so easier to pack round other stuff. Good for trips in the UK too - less space taken up in the car.