The Spendor S3/5 is a nice speaker, but it is at the lower end of their range (and it is not a duplicate of the BBC LS3/5a.)
I've got a pair of SP3/1Ps and a pair of SP1/2Es. While the 3/1s are very nice speakers and one can easily spot the family resemblance, there simply is no question that the SP1/2Es are far superior in terms of tonality and accuracy across the board, including the midrange.
If you basically like the sound of your current Spendors but are looking to improve on what you already enjoy it makes sense to audition the S3/5's big brother.
Of course, you've received some excellent suggestions for other brands so you've got a lot of good options to consider.