A bit late to this, sorry, but a plastic "cork" such as one by Nomacorc (other makes are available) just isn't designed for long term cellaring - you get a shelf life of perhaps two years at the most. If it wasn't brand new when it was given to you then it's possible, likely even, that the closure had simply reached the end of its life already.
It is an incomplete truth that wine improves with age. Certain types of wine, in bottles with proper (i.e. not recycled or agglomerate) cork stoppers, have bottle ageing potential when stored in good conditions - low fluctuations in temperature, in the dark, on their sides to keep the cork wet. Because cork is a natural material, there is always the risk of fungal contamination reacting with the cork to produce cork "taint". As a PP pointed out, "corked" wine is very distinctive - musty, mildewy, like mouldy newspaper or something. It is quite different from one which has simply been left open for too long and has turned to vinegar. But because natural cork is the only type of stopper which allows the tiny amounts of oxygen needed for in-bottle ageing to take place, and because it is a requirement of many European DOC rules, we continue to use it for very top quality wines intended for cellaring and have to accept that a proportion of bottles will become tainted.
The vast majority of the wine available in the UK, particularly anything from a national supermarket or off-licence chain and anything under, realistically, about £15 a bottle, simply isn't in that category to begin with however. Most wines nowadays are made to be drunk within a couple of years of production (not least because the storage costs to the producer are potentially huge) and because ageing isn't really a consideration alternative stoppers to natural cork are used to eliminate the risk of corking.
If it has a screw cap (which lets in no oxygen at all), it won't improve with age and although it might get a bit flabby as it gets older it shouldn't really spoil if the seal remains intact. If it has a plastic stopper (which lets too much in, albeit imperceptibly), it won't last more than a couple of years regardless.