Well, a surprising turn of events.
First the garage turned up to load the car on the flat-bed only to turn the key and drive it on there. What a muppet! I said if the chain went that's a new 2.2 in there at their expense. As he was creeping forward to hit the stop on the flat-bed, bugger me the noise ceased. I initially thought that was it, bang, but the engine was still running. Hmm. :wtf:
Upon inspection, the chain is fine and now they can't replicate the noise. The assumption was stuck lifters due to oil starvation because the oil was mank so they're doing a complete flush and putting fully synthetic in it. Hopefully, once topped up and changed it shouldn't tap when cold and I never have to hear that death rattle again.