Seals on cassette loo

If the mackerel idea doesn't work, and you aren't really into toilet DIY then a Thetford 'fresh up kit' for under £100 if you search, might be worth considering for cassettes a few years old. The kit comprises of a new cassette unit, toilet seat and lid along with a couple of bottles of toilet chemicals.

Yes, but they vary.  Repalced mine a couple of weeks ago and it was as you describe BB.  First had to take out the 8 screws then turn the casstte grey sliding cover (at last a use for it!)  the wrong way round and use it as a lever to twist off the sliding mechanism to get access to the seal.  Once you have removed the mechanism turn it over and remove the screw which holds the sliding cover, and remove the slide, to get access to the seal.  When refitting spray all wiith silicon spray to aid fitting and operation.  The hardest part was twisting off the sliding mechanism as I was worried that I might break the cover.  I now have a very clean cassette with a good seal!  As I said above, this is all within the cabilities of most of us.  On some models you just remove the 8 screws (4 may be under the 2 covers) and then lift out the seal and replace.

I had to replace mine, but there are no screws that need to be undone. It took a bit of brute force & ingnorance to get the old shutter to move, using the slide mechanism. 

{  I  was  going  to  say  why  do  you  let  the  seals use  your  Loo,  but  I  really  shouldn't    !  ) 

No ABM, you must not say that. I had a good chuckle at your comment though!