What was the date of the 1981 British Open squash final between Geoff
Hunt and Jahangir Khan. This was the last match Jahangir lost for over
5 years. I know it was March or April but can't find the exact date.

This is the only information I was able to find:

"Jahangir Khan dominated the squash world through the eighties and
into the nineties, winning a record 10 British Open titles and six
World Open crowns.

He established a remarkable record - believed to be unequalled in any
sport - when he maintained an unbeaten run for five and a half years,
winning countless matches in events all over the globe, from 10 April
1981 until 11 November 1986!"
http://www.eventis-sports.com/squash_awards.htm

I hope you find this helpful.

Best regards,
Rainbow