In so far as I can remember, there wasn't anything elaborate, just a thick rectangle of steel with a tapped hole in it for the locking bolt to pull it tight against the underside of the bed.

These holes are for two separate pieces, not one.

Your L5 might differs in some details from mine, but I have tried to investigate this from your photograph, the Operating & Servicing Manual and my own L5 – using a torch to see what's hiding under the saddle.. The diagram is not too informative and does not show the Locking and other screws and pins.

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Those holes hold two blocks called the Front Strips, Left-hand and Right Hand; the latter having a rebate in its upper surface, both having a row of three holes according to the diagram.

These act on the underside of the overhanging shear to prevent the front of the saddle from lifting, and the locking screw must be tapped into the Front Plate, RH.

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The smaller holes – the outer ones for the FP, LH (top on your photo) and the single one you marked with a red square, are for dowels / guide-pins set into the Plates.The RH plate is the locking one, the LH plate is held with a hexagon-headed set-screw from underneath into that tapped hole.

If you look carefully at your photo, you can just see where the plates were by very slight colour changes.

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I am not sure what the rebate in the Right-Hand Front Plate does, only that it faces upwards, to the right and the front – which would reduce the locking area.

I could not see this rebate on my lathe – though I might have replaced it wrongly having had to dismantle the machine to move home! I have noticed if I am a bit too heavy-handed engaging the feed with the tool already on the work, I cause a slight dig-in by lifting the saddle, but once hands-off the lathe runs as it should – so now use the control delicately, pinching the lever and trigger between finger-and-thumb and easing it upwards.

A pleasure! Do tell us how you get on.

The locking-screw will be of BSF or BSW thread to be compatible with the rest of the lathe. That holding the left hand plate will almost certainly be one of those anyway (unless someone has modified it with a different thread).

I think you'd need put the saddle onto the ways without the apron so you can measure the plates' overlap to reach under the shear without touching the root; but the rest should be straightforward. They look on my lathe to be about 1/2 or 5/8 inch thick, but I've not measured that. The marks on the casting will give you their lengths. The left-hand one ends flush with the outer face of the saddle.

The locating pins would not need be anything too sophisticated because they are not heavily loaded. I'd use a short piece of silver-steel or stainless steel for the locking-plate guide-pin – not hardened – but the ones in the fixed plate could be mild-steel or perhaps roll-pins.

What’s the problem with the clutch?

I’m not sure if it is also a brake, but I have to be careful on mine as I have mounted the motor on a frame on the wall above the lathe, and if I’m too enthusiastic releasing it, it knocks the belts off. (My design error, not its!)