Looking at Autotrader UK, there are 59 blackbirds currently for sale in UK. Most have lower mileage than yours. I'd say a ballpark is £2000 to £3000 using those others as a reference. But your best bet is to wait for someone from the UK to answer.
I'd agree with Leigh, compared to what's available, 2 to 3K. The higher priced bikes are usually in the dealers, so your looking at the lower end for a private sale.
There comes a point when the price levels off and even though the bike gets older, the value stays more or less the same.