Hi Mr. Angry,
I have a soft spot for the Moody 33 Mk1 as it was the boat I first started cruising seriously in. It was a friend's boat, originallly called The Black Pig then changed to Scott Free. I went to St. Kilda in her, and later out to the Canaries. She was built like a brick outhouse. She had a single cylinder Bukh though, don;t know much about the Perkins -although we do have the manual for it online here in PDF format
The back pressure in the sump sounds like rings - you might want to replace them yourself. It's not rocket science, and you could save a huge amount of money plus have the satisfaction of knowing the job had been done properly.