OK @swisscheese, to help you, I'll go into detail;
Here are the calculation for the depths of the working platform thickness provided by TC Vibro (I'm working with Dan and John Ashton the Contract Manager)
PilingMatCalcs.pdf
The numbers which interest us are the Required Working Platform Thickness (bottom of page 1). There's one for 'raw' thickness of 0.48m and the other for a piling mat with Geosynthetic Reinforcement , 0.30m
What constitutes a piling mat if you need new fill? Here's the answer - again from TC Vibro
Specification for Upfill Materials.pdf
Are there any guidelines for Piling mats? Of course there are.
BR470t.pdf
In preparation for this exercise we had a topographical survey done (5 locations) soil sampling 5 locations and a site visit done by Dan from TC Vibro. He asked me to dig a three meter hole in front of him so that he could hold the clay that I dug out. He needed that done in front of him so he could judge the water content.
The charges for the rig set up are eye watering (£2.5k per visit) but after that each stone column of stone is relatively cheap. As I explain above -broadly- there's probably no need to get the mat tested. But on balance, why would I not demonstrate good faith to the company when they have visited me three times (Dan twice and the Contracts manager once) to make sure the risk of failure is as low as we can reasonably get? I also visited a site on which TC Vibro were working.
In addition I have called out the water board to survey the water pipes (compulsory when piling within 15 meters of a water pipe here), but more honoured in the breach than the observance I think. Luckily, our pipes are cast iron, and a meter below ground level.
Can I suggest a bit of care with the crushed concrete spec (see the Up-fill spec attached). Dan informally talked me through some of the poor practice that some contractors do in preparation for the piling mat: the end in mind is making sure the piler (or in our case the 'poker') can get through the mat and the substrate. If the area hasn't been dug to the correct depth, the risk increases.
@John Ashton, you may want to comment on the above.
@swisscheese, I'd be interested in a bit more detail if you can bear it.....