The serial number is 43440. I'm pretty sure the xovers and tweeters are the
originals. The cabinets are in great condition.
The 43xxx serial number identifies them as manufactured in 1988, so fairly early ones. The mkII mid/bass driver upgrade would have been done during or post 1995, these drivers tend to last quite well so we can assume they are okay...
If the x-overs and tweeters are originals from 1988 they will be well past their best, and the performance you'll hear won't represent anywhere near as good as their potential - in fact they will probably sound awful. However, if the cabinets and drivers are fine then you have the option of replacing the worn out bits and bringing your SBLs back up to spec, it maybe a leap of faith if you've not heard SBLs because you'll need to spend some money on them but IME this is still a very cost effective way of gaining a classic speaker that has the capability of out-performing any other speaker for 2 or 3 times your total expenditure.
You live fairly close to Rearsby, Leicestershire - the home of Cymbiosis (naim dealer) Peter there is a good engineer who can replace your tweeters with a new matched pair of naim supplied ones, not sure how much this costs now but will guess around £400 ish, plus x-overs are still available for SBLs at £228, and a new gasket set too which was around 80 quid last time i looked.
Spending over 700 quid on an old pair of SBL may seem daft but in reality you'd need to spend a lot more to get a better pair of speakers, and if hypothetically SBLs were still manufactured today they'd probably cost over £4k a pair [or more?]
Also recommend you use naca5 speaker cable - if you're not already, and at least 4 meters long for each length : )
hope this helps