Calm down, calm down! what you're experiencing here is a common problem with all used cars in cold, wet weather. It's nothing to worry about.
First of all, stop using the redex - cars don't need that sort of junk
Going on what you have said, you've probably got duff spark plugs, or plug leads (also known as HT leads). Spark plugs should cost about £15 for the set, plug leads will be about £40 - but you may not need these.
The only real way to decide which is at fault is to replace them. My guess is the spark plugs.
The symptoms are caused by fuel not being ignited, either by spark plugs that are corroded/worn/carbon coated, or plug leads that are damaged.
Normally, this problem manifests by the car chugging/spluttering at low revs. If you thrash it / keep the revs high it doesnt splutter as much. A quick fix is to get some WD40 and spray it over the bit where the spark plugs are sunk into the cylinder head (dont remove the rubber, just give it a good soak). Also blast some at the other end of the plug leads (distributor) and generally coat the cables. This will push out any moisture and create an electrical bridge in any broken bits in the cable. My car did what yours is doing, and I was able to put off fixing it for about a month by doing this twice a week.
If you arent mechanically minded, do some googling on replacing spark plugs (it's a doddle btw). If you get some of the Iridium triple/double tip type you wont have to worry about setting gaps.
Take it easy
