Leaking Slime inner tube valve after opened for first time since install I put some extra air in my rear mountain bike tyre yesterday. It uses the SLIME inner tube. It wasn't flat nor had a puncture, I was just topping it up to make the tyre a bit harder.

I cycled to work on it and all was fine. When I came out of work however, the rear tyre was completely flat. It was absolutely fine when I got to work though.

It appears that since opening the valve to pump in some air, it's been slowly leaking back out of this valve. It wasn't a problem until I did this. It's a schrader valve if of any use/relevance.

Is there anything I can do? I pump the tyre up and it just deflates again through the SLIME inner tube valve (which has spat out a bit of it's green gunk incidentally). Just seems like a complete waste of money at £16 per inner tube if it just leaks via the valve (regardless of whether it's going to seal any punctures or not).
You might want to remove your Schrader valve's stem (innards) with a stem removal tool and check the valve's rubber seat, located on the part that you remove. There may be debris lodged in the valve seat such as grit or sand or some dried slime that would cause it not to seat and seal properly. Also consider switching the valve stem core from another valve from say an old inner tube and see if that does the trick. Unless the valve was damaged or the old tube you take it from is real old, the valve stem core will usually last much longer than the actual tube. Many years ago when my bike had Schraders, I'd keep a couple of valve cores from old tubes in my patch kit in case I experienced a valve problem when out on a trek.