Erm............... I think it should be ok for your pot plants. Might wanna test the Ph though. No chance of moving the water butt i suppose?

didn't realise you were a new member Welcome my friend.

Should be fine to use deepsoul. are the leaves from the conifer falling into it and fouling it? there could be a lid to pop on to stop leaves falling in. It may be slightly acidic but shouldnt be a problem.

Hi Deepsoul and welcome to GoY, the conifer is not the reason that the water in your water butt is smelly. The longer you leave it the more stagnant it becomes and starts to grow bacteria.Unless the butt is covered it is also possible that something has fallen in and died and is decomposing... Neither of which will bother your flowering plants.

When did you last clean out the water butt. I empty ours out every few years and hose the gunk out of the bottom. The should be no harm in using the water as it is, it will just smell a bit.

I wait till we have a drought which is easy in Essex and rinse out the empty butt, as suggested by Bulbaholic. A touch of tar oil in a low butt, let it stay a while, wait for a good down fall, then decant to the drain. Helps to keep mosquitos down too. Do it the late autumn when you won't have much use for the butt.