The top of the photo is the front of the gearbox. The saddle clip just presses onto a small shaft; no tools required; your fingers will be enough.
 
If nothing else, this photo shows that we forgot to seal up the gear lever hole before doing a lot of grinding. Should be fun cleaning that out now.

Thanks Flighter,
I’ve had a look at my gearbox and also another one I have here and they both appear to have a plastic or nylon clip but it looks different to the one in your picture and also different to the ones I have ordered on eBay.
I have attached a picture of the 4 speed gearbox I have just had rebuilt and also one of my shifter. Please let me know what you think.

Best Regards,
Paul

I fitted clips like you ordered (although the plastic ones), for both 4 and 5 speed gear boxes, and they work fine. I suggest using some pliers to pull yours out, as perhaps a piece has broken off (I suspect so based on the look of the LHS), or it was put in the wrong way (although I don't know if that is even possible). A broken clip will permit you to select reverse without depressing the gear lever first; that's why I replaced mine.

Hi Flighter,
I thought that may be the case. Mine definitely looks like the piece of the saddle clip on the left hand side is not there.
( although there is no sign of anything being broken). It shouldn't take too long and the clips that I've ordered should arrive and I'll let you know how I go.
Thanks once again for your help and also thankyou to Drew as well.

Hi Guys,
Eventually got my rebuilt gearbox fitted and installed the fibre reinforced saddle clip, however I still seem to have something wrong. The first-second gate seems fine, across to third and fourth is really stiff and I don't need to depress the lever for reverse which is really close to first-second. Once into the third -fourth gate the lever is really stiff and reluctant to come back to the first-second gate but once it does, it's really easy to go straight through to reverse. I'm wondering if I have the plastic saddle installed incorrectly or maybe the shift lever is the wrong way round? Any suggestions would be much appreciated.

All I can add is that the plastic saddle goes in like Flighter's 'dirty' photo shows, & the lever goes in with the notch to the drivers side. 

Thanks Nono,
Would there be any chance that the left fork on the gear shift lever is not locating in the “saddle” and may be just pushing the saddle around on the shaft ?

When the shifter is installed can it be lifted when pulled straight up? 
There is a spring on the shifter under the rubber boot that may be broken or dislodged allowing the shaft to sit too low down, essentially where it would be when depressed when selecting reverse.

Thanks for the info blue74,
My lever definitely doesn’t seem right. I took it out of the car and put the bottom into a vice and I can’t really get any movement either up down so I think you may be right, it seems my lever is stuck in the depressed position for selecting reverse gear. Can the lever be pulled apart and rebuilt with a new spring, or am I better off buying an aftermarket shift lever ?

Best regards,
Paul

Put up a photo of your gear lever.