Thanks for all replies. We pulled the valve cover off and found a tell tale pink (fuel) at several of the injectors where they are threaded into the block. Perhaps the little copper washers used there had been re-used....?
But on disassembling the fuel pipe (which snakes through and connects to the injectors, it CAME APART into two pieces. Suspicion is that it had been torqued down without an even distribution of torque, i.e. that rather than the recommended 'little at a time' method of tightening the connections, some part got tightened down prematurely, and with subsequent vibration it came apart eventually.
Anyway, we've ordered a new one and should have tomorrow, and hopefully that and a couple of fresh oil
flushes, will be that.