Like most of the other reviewers, I had a problem with black spot mould on the window sealant in my bedroom. Even though the window fitters used anti-fungal sealant, there's only so much it could take before the nasties started to grow and multiply. I'd tried a number of ways to shift the mould, ranging from kitchen and bathroom cleaner to neat bleach (including the icky-sticky Domestos Grotbuster) with little in the way of success.
Chemically, there's nothing particularly special about the HG Mould Spray - it's sodium hypochlorite (the main ingredient of bleach) and sodium hydroxide. You spray it on, leave it for a while (10-15 minutes) and then sponge it off. And guess what ... it works! It might take several applications to shift really stubborn mould, but after a couple of applications the sealant on my bedroom window frame looks far, far better than it did in that you can now tell that the sealant underneath is actually white!
There are a few gotchas though: first of all, it's fairly nasty stuff, especially if you have sensitive skin, so a decent pair of rubber gloves is an absolute must. Also essential is ventilation, because this stuff really, really stinks - it's not an offensive odour, but it is like bleach to the power of ten. It won't attack silicone sealant or uPVC, but it *will* make short work of any shoddy paintwork if you leave it on for too long - it'll also bleach fabrics, so keep it off your curtains or upholstery as well.
If you want to keep things clean and mould free, go straight for this and accept no substitutes - it's that good.