2.3-16v Weird Power Loss

Hi all - I've posted this on other forums, but to no avail... so your turn, see if you can figure this one out.

1986 UK 2.3-16v Manual c/w 84,000 miles
Running on 98RON Fuel, with octane booster

It just won't make power - runs as sweet as a nut, doesn't miss, lurch or anything. Really reluctant to rev and is probably producing less than 100bhp.

I've done the following:

New Fuel Distributor (which made it better)
New Fuel Lines to Injectors
New Injectors
New Air Temp Sensor
New Coil
New Dizzy Cap
New Rotor Arm
New HT & King Leads
New Fuel Filter


Compression fine
Valve clearances bang on


Any clues? as this is starting to cost a lost of money!

Thanks

Dunk