By the sound of it you have some type of aftermarket seat.

AFAIK ALL OEM Mini seats adjust for leg length.

I have the referenced brackets for my Cobra seat but it may not fit whatever you have.  However, as ve9aa suggests, once you get the position right you never need to adjust it again.... (So you might as well fiddle around and bolt it in the right place to begin with :)

A picture (or 2) would be useful if you're still looking for suggestions.

Cheers, Ian

 

Thanks, folks. I was thinking of something like these:

https://www.7ent.com/products/mounting-bracket-cobra-seats-adjustable-rear-feet-sse0002.html

That was funny Dan.

I "think" my current seats from a 92? British Open Classic are sliders......but even if they weren't, just buy the little adjustable brackets.
https://www.minimania.com/part/BG5001/Austin-Mini-Adjustable-Seat-Bracket-Extensions-Pair
That's what I did .  I don't remember now what they're called (I'll search this site after I hit send), but they are kinda L-shaped with a dozen holes drilled into them.  And instead of a bolt to hold your seat to the hinge, they have a pin with a slide-in clip (so you can do quick swaps). 

I tried my seat in a few different positions and when I got it where I liked it, I never moved it again.  This L-shaped hingey dooey brackety thingy puts your seat back a little bit as I recall.

"Hi sailor"???? (Sorry couldn't resist a joke)
As built, stock Mini seats do have a bit of adjustment. A very little bit. People who have installed seats from other car makes (e.g. Miata), which do have slide rails, have fabricated support brackets to anchor the rails to the cross-member the stock seats sit on. The problem then is that the new seats, though they do fit between the doors, run into the companion storage bins at each end of the back seat. Some modify the bins, but then you have to deal with seat belt reels and anchor points.

It might be easier to get used to driving with your knees bent, as per the original Mini design.

I’ve got a ‘92 with fixed (tilting) front seats. Anyone retrofitted theirs to sliders?
thaTIA. PT