Below is the reply from Ideal that I received after asking their tech department..
It is unfortunate that you have had issues with your local inspector because we worked closely with bodies such as the Electrical Safety Council and NICEIC well before putting these connectors onto the UK market. Both organisations fully evaluated these products and I can categorically state that the NICEIC (if that is who you are referring to) from a head office perspective have no issues with IDEAL's twist-on wire connectors. However, individual inspectors may harbour their own prejudices and these can usually be overcome by demonstrating the product to them and proving compliance to BS 7671. Loosening of the connector is unlikely to happen (assuming the connector is applied to the wires correctly in the first place) due to the conical shaped spring inside that grips the wires tightly.
I have attached a Q&A sheet that was produced to specifically address any questions an electrician or inspector may have, so it may be worth showing this to the person concerned.
Failing that, if you provide the name of the individual and the name of the registered body that they work for, we can take this matter up with them directly.
Rest assured that there are many electricians using IDEAL Twist-On wire connectors in the UK without any problems, but in this case it looks like we have a local inspector that is making up his own rules.