I don't think you will have much choice but to use a decoupling mat. Who drew up your floor spec? It appears to be defficient by one DPM and by the sound of it it is missing some joints as well. If you stick your tiles straight down I think they will fail.
The spec should inlcude 2 polythene membranes. One, DPM grade (1200g), over the concrete base and under the insulation the second, minimum 500g, over the insulation as a slip membrane. Problem with your set up is that over time the concrete substrate will give up its construction moisture very slowly. This will rise through the floor as it can't go downwards because presumably there is a DPM there. A DPM on top of the concrete will trap this moisture which will, as it is cement based do no harm whatsoever. The moisture will over a long period be used up in the hydration reaction with the cement and the rate at which it does penetrate the DPM will not be fast enough to cause a problem. with no DPM it rises through the insulation layer and hits what was a DPM but is now, thanks to the numerous penetrations by the underfloor heating clips, a seive. This moisture holds the potential to disrupt the adhesion of the tiles.
Additionally there should be joints in the screed running across door thresholds and also to separate any independently controlled heating zones. Bay sizes in this instance fall well within the maximum sizes recomended either by the anhydrite manufacturers or by the British Standards so at least that is not of concern.
At 60m2 the level of thermal expansion is likely to be negligible in the screed.
It is fair to say however that the tiles and adhesive will expand and contract at a different rate thus presenting a potential mechanical stress at the interface. If this interface is weakened by the presence of moisture your tiles will pop.
If he was saying that you need a decoupling mat just because it is anhydrite, then the chap from Topps Tiles was talking "Ballcocks". A decoupling mat is not essential on anhydrite screed. Provided it is designed and installed correctly and you use the right materials over the top it is not a problem sticking directly to it
However if he is more astute than I give him credit for and he was referring to the specification then he was absolutely right.
The adhesive over the decoupling mat is largely irrelevent to the screed type as it is completely divorced from it. A flexi adhesive will be suitable. It does not generally matter if there is some localised delamination under the decoupling mat but obviously it should be stuck down fully so I would be looking at a water dispersible epoxy primer and a flexible cement based addy in this particular case.
Also your screed is deeper than it needed to be. In most underfloor heating systems you only need 30mm cover to the pipes hence 50mm nominal is usually sufficient. The extra 15mm equates to 30days extra drying time.
Don't forget that you need to comission the underfloor heating system before you start to tile. It is not really likely that it will crack but if it is going to do so it will do it at this point. You don't want your tiles in place if it does crack. It is most likely to go at door thresholds hence requirement for joints.
Homelux, ditra, g-mat and durabase all perform exactly the same function. use the one which offers you the best value for money. Topps are presumably not offering you a guarantee if you use Homelux in preference to other mats.
As Grumpy says PM wetdecs cos he supplies several different ones.
