I finished it last night. Lamentably, I had to resort to a guide for a couple of puzzles that were proving to be real bottlenecks: Getting the squeaky mouse toy off the cat and using Fred the Mummy in the beauty contest. I had been thinking along the right lines for the first one, but I expect the correct method would have remained elusive. I thought I needed to stop the arty brothers working, so they would go to their room and play with the cat. Instead, it turned out to be one of the very, very few times in the game that I had to use a specific command, that wasn't the default, on a world object. Up to you whether that was the me being thick, or the game not communicating its mechanics well enough - but it's definitely the latter, since the game is littered with otherwise irrelevant interactive scenery and the original interface is hideously overcomplicated - I'm supposed to go around clicking Push, Pull and Use on everything, on the offchance that one of them actually does something?. I can more easily accept The Mummy one as my fault, since I had already used it in another puzzle (although that one was screamingly obvious in comparison). I might have made the leap more easily if it had also been the solution to a past puzzle.
I managed to defeat Purple Tentacle without even trying. The way his shrink ray pinged when it was recharging, I naturally assumed the solution would be to trick him with reception bell. I was just going through all the dialogue options for fun, when I blundered into the the real answer.
Overall, some gripes aside, I had a good time playing it. Whether I would be as sanguine about it had I paid 50-odd quid for it back in 1993, I don't know - but I didn't, so I am. I may not be a full convert to the genre just yet, but I'm certainly not averse to getting some more adventure games (when they are next on sale). In the meantime, I can give Maniac Mansion a go.