Ok, its been a wee while since I posted this & the engine has been sitting staring at me every time I go into the garage for the last 2 months. I think some of the replies above may be for an earlier model than mine, as mine is of the 5-6-8 Hp type of (I think) 1997/9 ish but..... tonight I discovered one of the pleasures of buying ex hire stuff!
Under closer examination, the "manifold" looked in better nick paint wise than the rest - so had it been replaced already? a closer look turns out the PN on the "manifold" casting is for the 6HP, and not the 5 the cowl proclaims it to be!
I then find under further examination that the throttle cam follower doesn't appear to be going all the way to the end of the cam - the spark advance plate is stopped a few degrees short of the roller hitting the top of the cam by a big lump of metal poking out the top of the manifold casting. I suspect 5 mins with a coarse file will allow it to go all the way & fully open the throttle. Also explains the one ignition system part number - it's physically restricted rather than electronically, and the power kept down by retarded spark.
I'm off to revisit the manual, as filing off too much will bring it out in sympathy with the Merc...