Don't buy a lotto ticket just yet, but perhaps set aside a dollar or two in prep.
I noticed your post, and remembered that I'd bought a new TXV valve for my '97 about three years ago when I was rebuilding and reinstalling the A/C ripped from my truck by a previous owner. (I actually tracked down all the original parts back then with the guy who bought it all after said removal. He'd hoped to install it in another vehicle, but the guy who pulled it all pretty much hosed him, as he'd destroyed three of four houses removing it and didn't send along the compressor. It was still installed making the serpentine setup work.)
Anyway, I stumbled across this part listed on Ebay back then and picked it up for about $30 as I recall, just in case I might need it, but didn't. It was listed as the TXV for my '97's A/C kit, and I expected that my stock one wouldn't work, but it did.
I just dug though my tool box and found it, still in the wrapper. Its not labeled with the STC3981 part number you and Neill mentioned, but it is marked with part number "1394712 - A/C EXpansion Valve", which cross-references to various Volvo, Jaguar and Rover uses online. It also looks identical to the TXV photo I found listed for a 110 kit, and as I remember, the one in my evap unit also looked identical when I was rebuilding it all and putting it in. Perhaps it's the same one or will work? I would not be surprised to find that the same valve is used in all sorts of 80s/90s Euro A/C kits.
My A/C is fully installed and operational and as such is not so accessible to check, but do you have a photo of your old TXV or can you take one? I'll be happy to compare.
Also, while trying to help out, I did search for STC3981 online and found a newer Rover part number JQD100470, which seems available in a few places if this one ends up not being what you need.
Hopefully this will help you out, one way or another!
------ Follow up post added December 3rd, 2010 11:16 PM ------
As I posted, Steve beat me to the new number.