The first week of May 1990 was the hottest start to May during the 20th Century. With high pressure nearby and under brilliant sunshine, temperature soared to 25+C in many places even as far north as Scotland. RAF Lossiemouth recording 27.4C on the 1st, Rothes in Scotland 28.3C on the 2nd, 28.6C at Brabourne (Worcs) on the 3rd and 28.3C at Cheltenham on the 4th. All these values are record maxima for their respective dates. The weather did become cooler and unsettled by the 6th.

It was a stunning spell of weather, I remember getting sunburnt on a college trip to Ingleborough

Wasn't there another significant heatwave in the first week of August on that same year? Where it reached 35 C in many locations over the south east.
  And all that came after what has been a particularly wet and stormy winter (quite akin to winter 2014).

Yes, August 3 1990 was the hottest day on record before 2003, with 37.1C (I think the official max was Cheltenham, not as mentioned in this BBC piece: http://news.bbc.co.uk/onthisday/hi/dates/stories/august/3/newsid_2527000/2527311.stm )

I remember I was on a scout camp in Devon at the time. Spent most of the day cooling off in the river next to the campsite.

Whilst not remembering the dates, I do recall May being uncomfortably warm. I also remember some warm/hot spells around the start of July, too. I changed jobs at the time, and had a week between finishing the old one & starting the new. Me and a few mates went to the Lakes for a day, and it was sweltering - we were swimming in Grasmere to cool down.

Kev, what was your MSI for that year?

I'm guessing not as high as 1995 from that era, but top 10?

Believe it or not not even in the top 25

I remember the warmth of early May 1990 also, was 10 at the time. I seem to recall that we had a lot of the same type of weather in May 1989 as well, especially later on.

June 1990 was a pretty poor month though with many unsettled days and below average temps, and early July here wasn't great but mid-late July and early August 1990 were fantastic.

Blimey! The poor spells must have been substantial to counter what I remember as good ones

About two weeks after the August heatwave, there was an absolute shocker of a day, a virtual gale, frequent showers and a maximum of 13.8C. Lancashire were playing in a Natwest semi final against Middlesex at Old Trafford. It took three days to finish it, the second day was a washout.

That must have been one of the largest changes in summer from good weather to bad weather in such a short time.