@agent – almost certainly something has got onto the disc. If the usual fixes don’t work, get medieval on them. I had to take mine off, sprayed them with disc cleaner then set it on fire. Did that a couple of times to get rid of any traces. For good measure I then sanded them. What ever was on there is likely on your pads too now so sand them too. You’ll have to bed them in again.
@kimbers – Guides are pretty easy to take apart if your oppo is even basically accomplished taking things apart and reassembling methodically. The internals are on Amazon for about £20. There is a special tool you need (get your mate to read the service manual). On eBay for £8.