It's such a unique and lovely thing ... and left handed ... so you are looking for a leftie that is willing to pay the right price and own the rifle.
It's also a full length 77 and not the K. So people who may want to use it for target would want the k version ... and no one is going to want to take an hacksaw to it. Maybe no one would want to drag that around a course anyway.
I like my target comp guns to be scarred and ugly ( like me ).
I've just roughly worked out in my head what it would cost for a standard rifle and to have all the extra bits done/added on ... I reckon you are around a 1000 quid ( maybe more ... sorry if I'm way out ). The stock alone may be worth more than my estimation of the stock's cost. The markings are lovely. I had little idea of the engraving.
You may get more replies re value if you could give an idea of what the tuning, Mamba sleeve, stock and engraving cost. Probably a bit cheeky and personal ... but it's you that is asking for an estimate.
So probably in the 6 - 700 quid selling on price.
It's just down to demand and what you can get ... and what you would let it go for.
Unless you have a serious collector interested I don't know who would pay that sort of money for a s/h springer.