It is way to early really to be wearing winter rubber on your car. Surely you must have felt the lack of traction in the warmer months? Unless you are driving miss daisy, in which case, why drive a GTI, you loose a lot of performance with winter tyres in warmer weather. Plus you generally rip through winter tyre tread when its warmer.
You COULD change all tyres to the likes of Michelin Cross-Climate tyres, which are meant to be very good in both summer and winter. You will loose a bit of performance against summer tyres in the summer, and the same against winter tyres in winter, but then you only need to run one set of wheels. But presumably you still have 3 good winter tyres, and you could sell them to recoup some money.
I would, personally though, change all the tyres to summer tyres, and buy a second set of wheels (second hand) for the winter tyres (or keep these wheels for winter and get new summer wheels). Although you are just about ready for winter tyres now, it's still not quite there. So it would make sense to keep summer rubber just now, as winters wouldn't really be needed for a couple months still. The advantage with running 2 sets of wheels is that it doesn't cost you fitting and balancing twice a year to swap the tyres over. And if you buy the wheels second hand, they will, for the most part, always be worth roughly what you paid for them, if you sell them when you sell your car. Then, you use the winter tread in the winter, and summer tread in the summer, saving both sets of tyres in the months where they are not suitable. This is, of course, assuming you have somewhere to keep a second set of wheels (I know not everyone does).
Otherwise, you could just put the winter back on just now, and look to getting summer rubber all round later, around March / April.
Surely you only need to replace one tyre though, rather than a pair? Especially if you just replaced one of the tyres a few months ago, and don't do a huge amount of miles? It must still be almost new, and so would match up well with the new tyre. No need to change the pair. And £300 seem a bit much for a pair too. Assuming they are 225/40/18, they are just over £100 from Camskill (
http://www.camskill.co.uk/m11b0s1316p130034/Dunlop_Tyres_Winter_Snow_Car_Dunlop_WinterSport_5_Dunlop_Winter_Sport_5_-_225_40_R18_92V_XL_MFS_TL_Fuel_Eff_%3A_E_Wet_Grip%3A_B_NoiseClass%3A_2_Noise%3A_71dB), and shouldn't be much more than £10-15 for fitting and balancing.
