CarBuff

Perhaps, try TWO lotus gaskets? In the USA I can go into a parts store and ask for a thermostat gasket for a 283 C.I. 'small block' Chevy V8 (very common here). But, in the Netherlands...Hmmmm.

Pack it well, @Paul Coleman ! They DO break.

Our '88 (Stevens) has a low vacuum switch near the servo that illuminates the BRAKE warning light on the dash. I once knocked a fitting askew over the gas tank and the light went on. Not to mention, the vacuum pump started complaining (noise). That was where the pipe split (heater to one side, boost transducer on the other)

The vacuum system is easy to 'half-split'. Minimum vacuum is 18hg. at various points. Plug the lines half through, see if that makes a difference; go forward or backward from there using the diagram under the hatch. If your car has the external vacuum pump, common trouble points are the rubber T near the front of the engine; and the elbows on the hard plastic lines. The ones located under the dash (heater controls), and also plumbed over the RH gas tank and sail panel have been known to have duff sealing capabilities.

"Some of our roads are quite bumpy here" Same here in the USA. In fact, I want one of these stickers for my Daily Driver. (Sorry for the off-topic post, had to share. )

The Bosch FI engines ('86 - '88 in North America and Australia) only used mechanical advance. The vacuum capsule was only used to increase engine speed during warm up.

In the USA, a good place to check is a dealer or repair shop for RVs (recreational vehicles, "caravans", etc). Or, a shop that converts ambulances... These vehicles often use custom-length seatbelts. A USA owner near me tried to replace a G-car belt with one from a Stevens car and they were different FWIW, if you were considering buying one from a breaker.

This is a straightforward repair. I THINK some folks used a sealer on new o rings, too. (Are we all afraid? LOL) It is always a good idea to use some sort of lubricant on newly fitted o-rings during assembly. Don't use anything that will affect rubber (petroleum based stuff is a no-no.) This stuff in the link below is specifically for the process, but I just use a thin smear of Sil-Glyde. I'm sure there is something similar in your locality. https://p80lubricants.com/2017/06/16/the-best-solution-for-easy-automotive-rubber-assembly/

Someone MIGHT have the 'boxed set' of Citroen Special Tools for loan on your side of the pond. I know of two sources in the USA.

Be careful when sourcing exhaust systems from the UK. Many will differ from USA spec. (eg, the tailpipe exits at the left side, or the catalyst/EBPV is different, or the piping diameter is different) The factory Bosch (Langford) exhaust is a 3" system. And, it is stainless steel. Sounds good. While you have the drivetrain out, fit a heavy-duty input shaft circlip (available from JAE or Harry Martens) to the transmission...

The bolts will be TIGHT from all the heat cycles. And access will be limited. One friend welded up a 90 degree wrench by cutting it in half and re-welding it at a right angle. Good Luck...

If my carpets looked that good, I wouldn't fit my seats to the car either!

Another thought upon re-reading the Original Posting: There is no factory Blow Off Valve on an '88. Did a previous owner fit one? Might be a good idea to block it from actuating during your troubleshooting. I've seen a few BOVs that have stuck and malfunctioned.

That is not a valid Lotus part number for the EOM. You are looking for a number more like 'A082M6374F' or better, 'B082M6375F'. BUT, you may be onto something with the EBPV (potato valve). Many owners wired those open so verify that the valve is in the 'correct' (open) position. JAE Parts in the USA sells a spacer to eliminate the valve.

Engine Overspeed Module. Look for it on the left side of the coil 'box' (rear engine wall). Does the part number start with an A or B? PS: Yes it DOES work by cutting power to the fuel pumps, can be a violent experience.