Try doing all the maintainence tasks and minor repairs by oneself! 
I know ppl hate wd40, but if you open the boot, and spray it into the lock. the barrel will be vertical and it should do some good.
One can not “hate” WD-40. It is useful in just too many ways.
However, there are places where different products are more appropriate.
Locks require a dry lubricant to minimize dust and grit contamination.
However went a lock is seized, WD40 will do a much better job of getting it moving again, and then you can apply your magic grease
Sorry for the delay in getting back to you guys. I’ve been seeing your notifications popping up, but just not had the time to come back with a reply to your posts.
Anyway, having just joined the forum, it’s great to see so many people chiming in with their views. Just want to say it’s all very much appreciated, and it’s good to see the forum being so active. Good job! 
In reply… Mercedes-Benz…I avoid main dealers like the plague, not just MB ones. I’ve owned a '92 500SL since 2014, and the only way I’ve managed to get the car to the condition it’s in right now, is through doing 95% of the work myself. The club forum and it’s available technical assistance is invaluable too, but if the car needs to go into a garage, there’s a careful selection process I go through to decide on who gets the car, dependent on the work needing done. Never, does it go to a stealership.
WD40…yep, I love the stuff too. I’m aware that it attracts dirt to an extent, especially as it dries out and becomes sticky, but for freeing things up, it’s my go-to ‘tool’. Not sure if adding graphite after using WD40 would be a good idea or not, but I do understand that graphite is a much better lubricant than WD40. I’ve got a jam jar half full of the stuff. My late Dad brought it home from the coal mine he worked at. Lovely stuff in the correct application
My boot lock…I didn’t get to it today. Had a very quick look, and found the boot lid cover is held on with fasteners of some description. I didn’t know how they pop off, so rather than breaking something, I’ve left this job until I understand how best to remove the clips. If anybody knows…feel free to say!
Instead, I’ve topped up the power steering fluid today. The MOT mentioned a ‘slight weep’ from the pump, and when I checked the fluid levels, I found that it was just below the level of the dipstick on the reservoir cap, so it wasn’t even registering. Now topped up to the MAX line though with fresh Dexron 3, and I’ll keep a close eye on the level over the next few weeks.
Also, I removed the windscreen scuttle cover to take a peek at the infamous rust trap that can be hidden beneath. Very happy to say, that apart from a small amount of muddy deposits in a few places, everything is rock solid. Very relieved to see this. Spent a couple of hours cleaning everything up, especially the drain channel along the bottom of the windscreen, so it’s all free draining now, and clean as a whistle.
So, hopefully that’s everything covered for now. I’m hoping to check the strength of the coolant/antifreeze tomorrow too. Needs checking before our Scottish Winter sets in.
By the way…I tried the spare key in the driver’s door lock, and it opens it fine. There’s also mention of a new ‘XJ6’ boot badge having been purchased by the previous owner. Came across the receipt while sifting through the pile of paperwork that the car came with. No mention of a replacement lock, do I’m hoping the badge was just replaced for cosmetic reasons. I’ve seen a few of these badges becoming corroded, so it’s possible I suppose. Anyway, once I figure out the best way of removing the boot lid cover, I’ll try to free it off as has been suggested by you guys.
Thanks again