<< but I cannot find any ICAO reference to this. Can anybody help as to where the UK's requirements come from?>>
Interesting - I was trained to use precisely that phraseology (and did, many times) when working abroad with an ICAO Licence back in the 60s. If there is no reference now, presumably things have changed.
"Delay nor determined" was always used when, for example, there was bad weather and the usual system of calculating EATs could not be used - much as happens nowadays in the UK.