Yes, I've looked at that stuff. If you go on the concrete industry's website and look at the pages that talk about reinforcement they are quite clear that steel is the way to go for anything involving tension so large slabs subject to ground heave, like on london clay, its needed. Fibres are good for other stuff, particularly controlling cracking due to shrinkage and the two should be considered complimentary.

The concrete I'll get pumped in will have fibres in it, but the firm in questions still recommended 'fabric' (thats the industry term for steel mesh). I need type 142 apparently, 6mm wire 200mm mesh. and It shuld be at least 50mm from any surface so you shouldnt hit it with a drill under normal circumstances, a 2" hole in concrete is quite deep.

Looking at using sleepers or 75mm half-poles as a retaining wall rather than concrete, it'll look nicer and is quite cheap.