This is the stuff I used as cement. The company calls it mortar, but its more like runny putty. It does the job, but its rather messy. In all I bought 5 tubs of the stuff. It isn't cheap, P&P is ?15 regardless of whether you have 1 tub or 10, so I bought 5. In the end, all I had left was about 1.5kg, so not bad estimating...Overall price was around ?90!
Edit: Seems like the price is now £30 a tub and P&P is £30 also