If we knew the answer we could become very rich.
The euro has been losing ground to the dollar the last month, and this looks like it might continue with an ongoing rally in the dollar.
Whilst the Euro has strengthened over the last two years, sterling has devalued – which is a) why our holidays in euroland are getting so expensive and b) why UK manufacturing has been doing relatively ok considering the economic mess.
Clearly the UK government wants to keep the pound weak, especially coming upto an election. So I'm not convinced there will be any fall in the sterling to the Euro, unless the Euro goes into crisis mode, which could happen. But in most events, I do not see the Euro significantly strengthening against the pound, so in that case I would just pay the deposit and keep your fingers crossed.