Thanks Chaps!
In answer to the above queries:
It's an SL 350 with the later 3.5L V6 272bhp engine and 7-speed gearbox. Mercifully, I established only after buying it, that its engine number shows that it's not one of the ones potentially afflicted by problems with the balancer shafts...phew!
I've actually had some of the older SLs:
1)
Pagoda 280SL
http://www.bmwclassics.co.uk/gallery/index.php?spgmGal=Mercedes_Pagoda
Beautiful to look at like a piece of good art or jewellery but absolutely horrible to drive.
2)
R107 500SL
Again, absolutely gorgeous to look at but again (sadly) not a 'driver's car'. Thirsty engine which fails to deliver what you expect from a 5.0L V8. Also, despite its overall size surprising cramped inside especially with the hard-top on and I'm only 6'2". Taller people would struggle.
3)
R129 SL280 and SL320 (V6)
I don't seem to have any pics of the 320 but here's the 280:
http://www.bmwclassics.co.uk/gallery/index.php?spgmGal=Mercedes_SL280&spgmPic=67&spgmFilters=#pic
The R129 is great IMHO. These are such a quantum leap up! Superb build quality that makes them feel like they were carved out of rock.
Only minimal scuttle shake with the hard roof off and not too much with it off whereas in the models before it's intrusively noticeable.
The SL280 wasn't exactly a
slow car but it did always feel like that engine had to work a bit too hard for that weight of car. The SL320 was much, much better in that regard.
Yes, I too remember when these R230s came out and being simply blown away by the looks. We lived in Glasgow at the time and my next door neighbour got one of the first in Scotland - an SL500 - and I remember him paying something like £93K for it!
I shied away from a 500 because of the potential (or apparently almost inevitable) suspension problems at £1000/corner and opted to go for the SL350 because it has a normal 'simple steel' suspension.
The interior is just lovely in this car. Every piece just has a real feel of quality about it. As an American said in a YouTube I watched recently: "In this car's interior, if it looks like metal...it is metal!" And that really sums it up.
I've only driven it twice so far; once as a test drive before buying and then about 85 miles home. So it's early days. But, to compare with the Z4 3.0Si?
Well, they're very different cars; built for a very different purpose and obviously originally to a very different price when new.
The Z4 3.0Si feels and is faster. 0-60 in 5.6 vs 6.6s for the Mercedes. And obviously as a sports car a far more nibble feeling to it.
The SL has a far more luxurious, cossetting and expensive feel to it. It not only has heated but cooled/ventilated seats! It's alsomy first experience in an SL where there is no scuttle shake whatsoever. But it feels heavier too and less agile.
I think a good way to sum it up might be:
A couple of summers ago we took the Z4 down to the Black Forest and into Switzerland. It was great once down there and on their really smooth roads. Actually getting down there wasn't so much fun in the Z4! Long stretches of UK Motorways and stretches through France when not of Autoroutes were quite wary which I guess is inevitable in a genuine soft-topped, stiffly sprung car which has also been built with some mindfulness of weight and hence with lesser regard to noise insulation etc.
So, perhaps in an ideal world, if one could afford it, you'd travel down there in your SL and then once there swap to your Z4! :lol:
Alternatively, I guess you could just get the later version of the SL500 or even the AMG SL55! However, I don't know about you chaps but I have enough trouble getting to sleep at night already without having to worry about one of those! :lol:
In terms of price? Yes, you do get a lot of car for your money and there were some quite tempting ones at below £10K which is originally what I was intending to spend. But, I actually paid A LOT more for this one principally because of the mileage but also because of its history and that it was exactly the spec I wanted in terms of model, interior, wheels etc.
It has only had two owners. The first from new until 2016 covered only 9K miles in it! The second took it from that to it's current mileage. But, it has had 9 MB Service stamps in that time and now another Specialist service prior to me collecting it!
I've recently had to retire after a very rough year health-wise so I thought "hang the extra expense even if it is a lot more money get what you really want!"
We've just this morning booked a 12 day touring holiday to Northern Spain in it and then to drive on to our place in SW France for an open-ended stay and I can't wait! 
NB Sorry for the italics!