Thanks to those that have replied so far. I also found the Practical Avionics article, but I suspect they are US-based.
However thank you "eglnyt" - that CAP 670 document has exactly what I needed, and indeed confirms that NDBs within the UK are 400Hz.
Now I wonder how all those previous sims were certified or at least wonder why this has not been DR'd before?
Is there a similar document for Europe? Is that under EASA control now?
Thanks, GY