Hi Scott,
Probably Chiff-chaff but I can't really remember that far back. I can remember creeping through the undergrowth and around bushes trying to see whatever it was singing so that I could remember it; it was usually a Willow Warbler or a Blackcap, or might it have been a Garden Warbler. That was in the days before there were tapes or discs available.
I agree with Sue, try one per night and try singing/chirruping with them; but in the end there's no substitute for actually seeing the bird singing. A lot of people find that using a mnemonic helps, for instance the Yellowhammer is often said to sing 'A little bit of bread and no cheeeeese'.
Keep trying, it's worth all the effort.
Bill.