Tapecrete is best, if you can get it. It is rare and expensive here in the States as it is made in India. It appears it is easier to get in the UK though.

While I know nothing of brand names available in the UK, I use Quickcrete brand cement and their acrylic fortifier. The acrylic fortifier is a necessity if you plan to use any concrete in a vivarium as it makes it 100 percent water proof. Skip mixing in water; use the acrylic fortifier straight out of the bottle. It is better to brush it on in many thin coats over time rather than one thick coat. Mix only what you can use in 10 minutes per batch. You must allow a minimum of 24 hours curing time between coats. Once all your coats are complete, you must allow a minimum three week cure time in a humid environment. If you do not allow for proper cure time the concrete will crack and crumble.

One note though: Expanding foam does not make a suitable base for concrete based constructions. It shrinks and swells over time and will crack the concrete. If you do want to use the expanding foam as a base, just remember it will require a minimum cure time of a month in a warm, humid environment or it WILL fail on you in a very short period of time.

I have a few threads on here featuring projects in progress with this stuff. I have one in particular on Caudat.org (search for "Okinawa Project"). Also on Caudata is a user by the handle of "grius" He is the master of concrete use for vivaria and taught me how to make it work. My projects are absolutely nothing compared to his museum display quality builds.