My somewhat lazy assumption was that peak Thomson was unplayable during the 1974/75 and 1975/76 series against England and WI but relatively ineffective thereafter due to the injury that he sustained at the start of the 1976/77 series against Pakistan. Reality is a bit more nuanced than that.
The 1974/75 Ashes were his absolute peak of course; 33 wickets at 17, and lots of the action is on youtube if anyone wants to take a peek. Similar to Johnson in 2013/14 really; a perfect storm of an extremely fast bowler at the absolute peak of his powers against batsmen who just hadn't seen it coming. Fast forward 12 months, and he again took a lot of wickets in the 5-1 win against WI, but each of his 29 wickets cost an average of 28.6 runs. Maybe the WI batsmen were better than their English counterparts at dealing with short pitched quick stuff? Maybe he wasn't fully fit at the start of the series, as he was relatively ineffective in the first two tests.
In between those two series, he did OK on England's slower pitches during the 1975 Ashes (16 wickets at 28.6). And a year or so after the WI series, he was injured. After that, purely from an England pov, he was actually more impressive in 1977 than he had been in 1975. And he had a pretty good 1977/78 series in the WI, even when the hosts played their first choice XI against Australia's post-Packer side. Thereafter, it looks like injuries kept him out of the side as often as he was in it. But he finished in style with 22 wickets at 18.6 in the 1982/83 Ashes. Even allowing for the fact that he was 32, it's odd that he didn't play again in the next two years, unless injuries did for him again.
Mind you, if slippy meant Geoff Lawson, then I've just wasted 15 minutes of my life.
EDIT
Looking into this a bit, I've probably understated his contribution to the 1975/76 WI series. Maybe he wasn't 100% fit for the first game, but apparently he was devastating for the last four tests.