I do not think the bass in the picture is a B4. The B4 has rounded horns. Those are pointy. It looks like an LB-20 to me. But that does have a bolt on neck. Mmmmm. I've had both an LB-20 and an Ibanez SR700. Both are excellent basses but I liked the Carvin better. Whatever that bass is I like it.
Most carvin bases have been offered with rounded and un-rounded horns That's a B4, the predecessor to the B40 that was recently discontinued. It was also offered in kit form (the B40 still is), but this looks like a factory bass.
The B4 is a fine bass and very well-made. Except for the pointy headstock (not my favorite) I'd take it over the Ibanez, but I'm partial to Carvin and USA made basses where possible.
For comparisons:
-Here's a newly produced Carvin B40 on closeout from the factory.
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Virtually the same bass with a few cosmetic differences and no fancy wood. It's on sale for $850 after mandatory case and shipping, though it would probably be even more expensive to ship overseas.
-The nearest current production Carvin, the JB version is similar but with a different shape, and the Jazz style plate and guard, has a base price of over $900.
-I don't have my old catalogs anymore, but as equipped, the bass you're looking at probably sold new for around $8-950
-If you wanted bought the B40 kit from Carvin today the price would be $450 before shipping.
If you like the Carvin, I'd go for it. Carvin's take a hit on resale and $500 is maybe just a touch high for a used one here in the states, but you're not likely to find one at that price and condition in the UK very often and it'd probably cost you a good deal more to import a similar Carvin from the USA.