I can't help with 1988, I moved here in 1996 and have kept records since. Generally I buy in the cheaper months of summer and can get through most of the year with no top ups unless it is very cold. I don't get it exactly right but have managed to avoid buying at 'the top' with a bit of tactical ordering. The most recent purchase was pretty horrifying - 89 point something a litre but I'm OK to Easter so can forget about it.
Alongside the per litre price I have adjusted for cpi (I know, I know there are plenty other measures of real inflation but this is vanilla ONS stuff) re-based to my 1996 start date. The chart is interesting, yes its bad, but with general background inflation not as bad as it looks - 2007 to 2013 were almost as bad in real terms.
(and yes, I was miffed when the price of a gallon of 2T for my Vespa went from 4/11 to 5/02, a landmark. Only bought in half gallons of course, tiny tank, but back then garages did have 2T hand pumps where you set the mixture ratio on a brass dial - 20:1 if I recall - and it mixed it for you as you pumped the handle.)