Am I right in assuming they had an oil spill and get vandalised? Someone please put in context! It's an excellent sitcom.

If it ended this way it should have a couple more episodes that match more with the series style "calm, slow and funny".

IIRC it was done in calm and tasteful way but the show was starting to get criticised by some for being too nice and cosy and not reflective of the state of the country at the time. And if I'm right it was decided to finally give it that bit of reality which also provided a suitable way it to finish. If the show ended on another calm and cosy happy ending episode then fans would be saying 'Why did it end?' They were blissfully happy and so were we.' All four of the stars were in great demand and would've struggled to fit their other work in had it continued.