well I've fixed mine and returning here to post how!
I've tested the resistance on the fuel pressure sensor ( on the fuel rail ) and it was fine and within spec, the problem I had was the pressure discharge valve/ suction control valve - the thing that lives on the back of high pressure pump attached by allen keys- the values that I had for resistance didn't match what I was told is the correct readings, however, even the replacement valve had the same 'incorrect' readings, so maybe that changes based of the part code and pump used.
removing the valve can be done easily with engine in, however, the replacement part that I sourced through a local diesel specialist was actually longer than the original, so re installation required undoing the engine mounts from below the car and jacking the engine up and giving it a wiggle to be able to reinstall the new one- fiddly but much easier than removing the whole engine to replace the pump!
the reason it's now longer is that the original lexus part and pumps are actually discontinued from denso and to make things complicated, those valves are paired by part code to the fuelpump part codes, so you can't just take a valve out of a breaker car if the pump part code doesn't match.
knowing the part codes (black sticker ontop of pump and an etched code on back of sensor) I was able to get a valve for use on the newer revised high pressure pump- even the specialist who sourced this suggested the pump would be required aswell...
after a prime for 2 mins it fired straight up, so despite what lexus and specialists will tell you, you do not need to replace the pump at a cost of £1200. you need a £120 part.
if you happen to have this problem remember that the pumps and valves must match, so people breaking them sell the whole pump assembly, which given this is a common problem I wouldn't suggest saving money there, especially when you think that it will require the engine to be removed to even get the damn pump off!