Fixed this properly today... wot a fiddly job!
And my bodge let me down (one of my strings snapped when trying to release bonnet...but luckily i could reach up from underneath with car raised to operate the spring catches with my hands - phew!)
As rgwm advised, there is a black metal cover behind inlet manifold on the bulkhead held on by a 10mm bolt in top & 10mm nut on side (onto stud sticking out from bulkhead)...i dropped both the nut & the bolt when removing them cos its so tight back there (even with engine stabiliser bar disconnected & engine tipped forward a bit with a ratchet strap!), so beware!
i also had to disconnect the cables/pipes at drivers-side end of inlet manifold to get access over top of brake m/cyl to do rest of this job
behind cover are 2 plastic cams that connect the throttle cables [top cam] & bonnet release cables [bottom cam], i.e. connect the short cables that come from the throttle pedal/bonnet release lever to the long cables that go around the engine bay
the short bonnet cable is a solid wire, so isn't very bendable (whereas all the other cables are multi-strand flexible cables) which adds to awkwardness of this job. my short bonnet cable had snapped just near the cam, leaving its retainer in the cam. rest of my cable i removed from inside car, pulling it thru the lever & bulkhead. this leaves a rubber boot in the bulkhead which you need to get out - this is really awkward too cos you just can't reach it properly from the cabin... i ended up cutting it out from engine side & pushing it thru
when refitting new cable, you must start from engine bay side cos that's the only way the cable will go thru bulkhead! insert the retainer 'T' on end of cable into the cam (i stuck a brick on throttle pedal to move top [throttle] cam out of way to give access for this). tie a bit of string onto nipple on other end of cable, & push this thru the bulkhead hole & pull cable thru bulkhead & up behind pedalbox (this is tricky cos cable is squashed into U shape behind inlet manifold & wants to ping out of cam...guess it would be easier with 2 people, but one of you still needs to have skinny arms/hands to work behind inlet manifold in 1st place!)
lastly i compressed the bonnet release springs (tying them together across top of radiator) to provide slack in bonnet release cable. then i could just about thread the nipple-end of the short cable into the release lever under the steering column. i lubricated the cams & cables whilst everything was still accessible
result - nice springy release lever, & bonnet that pops up when you pull it (i also cleaned & greased the 'mushroom' catches on the bonnet, plus slightly realigned one of them)
but bit of a pig of a job this one really....
still, i hope above helps if anyone needs to try it themselves!