Blackmail: in this situation, how immoral is it?

I'd tell him he aint getting his stuff until you get your racket and just see what he says.

4 months after he dropped it off? The shop probably binned it/sold it by now.

I am not a philosopher, so I can't talk about the moral side of the situation competently, but I personally feel that your situation might be a useful lever to get what you want.
He wants something from you, he has also fullfilled his part of the pakt by paying the money so he is entitled to get the goods.
The racket is a different problem: here he owes you something, which he hasn't "payed" yet. The question would be: is the amount of money he has given you in relation to the loss of the racket? Could we then say, that he has thus "payed" for the racket and the "payment" for the goods you ordered would be the racket, meaning that if he returns the racket you will "refund" him with the goods?

I guess it also has to be considered whether this situation would endanger your friendship. He does seem to be reliable or else he might not have payed. Maybe he just needs a real kick to go and get the racket.

He paid me £50 for the stuff, but that simply covered the £50 which I paid originally to order it.

The racket is worth at least £100 (or would be to buy a similar racket brand new). Add to this the fact that I don't want his stuff, I want my damn racket, and this paying for the racket with the goods thing doesn't really work.

He's trustworthy with money and stuff, he's just really forgetful and lazy.

How about phoning the shop and asking for the racket to be sent? You could get him to pay the money by bank directly to the shop.

His excuse for it taking 4 months and counting is that he keeps forgetting.

How much do you want to bet that now there's something he wants out of it that his memory will suddenly improve?

To be honest, the weather is pretty crap at the moment and I'm not desperate for my racket, maybe if he picks my racket up I'll keep "forgetting" to drop his stuff off to him, and if he calls round maybe I'll keep "forgetting" where I put his stuff, see how he likes it.

Should I really have to do that when it was his responsibility to collect it? Then I would have to get him to pay the money by bank, and that's if the shop would even be willing to do that.

My view of this situation hangs on one thing. Who paid for the restringing?

Do you want the bl**dy racket or not? You have tried all sorts of ways to get it. I take it he is paying for the restringing and just can't be bothered to get it from the shop. Maybe he brought it there because it was less expensive or whatever reason. But since he is not moving you will have to think of alternatives. The main problem seems to be the drive to the shop, so maybe sending (and paying p&p on top) could be a way to please both of you. Why not suggest that to him and get him to get the shop's bank details, transfer the money and then they send the racket to you?
Otherwise you might just have to really tell him that either he can collect the thing or give you the 100 pounds to buy a new one. Set a date by which you want it back.

It would be nice of you to leave him alone, go and pick the bloody racquet up yourself and to shut up about it then. He paid for it when he clearly didn't need to, and you said yourself it wasn't his fault - they just sometimes snap. Why can't you go and pick it up?

Well the situation is a bit uncomfortable, if you are sure that he is not to blame for the broken strings. He does seem a reasonably responsible chap, but I guess you will really have to make clear that you want the racket back within x days.