Bit late now but for the benefit of others, tanking is absolutely not required!
These types of panels (xps) use a small amount of sealant between the joints of the boards. It is then common for a waterproofing liquid to be painted across the joint and a rubber membrane applied over the top. A final application of the liquid is then further applied to overlap the edges of the membrane. Jackoboards, Wedi, Elements boards all use a similar technique.
These panels are 100% waterproof, whereas all the other options are water resistant. When using these panels though you have to be aware of just how expensive the sealants, screws and membranes are. You can spend a small fortune on the stuff.
I have an untested theory that you could use the cheapest products available on any of the boards and it would be fine, but it depends whether you are brave enough to try it. For example, one manufacturer offers the sealant at £7 per cartridge, whereas Wedi charge north of £15 per cartridge and you are going to use a lot of it!
All things considered, this is an expensive solution but the best in my opinion.
I have used these boards for my bathroom and I will use them again, even though it at around £200 to the overall cost.
www.wickes.co.uk
