Where in the UK are you from ?? (if you dont mind me asking)I
It is entirely possible! A 1998 I would have thought should definitely have seen symptoms by now. There's an easy way to check.
If you are up for taking the amplifier chassis out of the head box and flip it upside down (just use a bunch of books to support it either side), look towards the rear of the amplifier on the PCB. You should see a white Marshall logo towards the top of that board with a little rectangular cut-out. Try not to touch anything, JCM2000s do not discharge their capacitors upon power off. If you need to move some wires out of the way, use a plastic or wooden poking device (a chopstick).
You are looking for a rectangular grey box printed on the board just under that rectangular cutout.
The most common board was Issue 5 (abbr. ISS 5). If it's a 5, you will have some fun and games down the line, it's really when not if. If you see 20, you're grand. As an aside, I would swear blind I have seen an ISS 22 board in the wild somewhere. Either I received an issue 22 from Marshall, or I am not remembering correctly and am making it up, likely brain fog over the years.
Hopefully that will help clear things up. I can say, any JCM2000 I have replaced the board on, I didn't get back in for a re-repair. Really, these are the prime sleeper amplifiers in my view. They are cheap to buy, if you really need to replace the mainboard, it is not terribly expensive to do and after that it's just as solid as any other Marshall I have had in over the years. Enjoy the bad rep and thus cheap price.