If I really really work hard, can I finish my PhD in two years? Many thanks for your help.

Depends on whether your uni will let you - some have set registration

Not likely as you will be paying 1 year fewer fees.

Also, it very much depends on your project. It's not all about how hard you work, sometimes there are things that are out of your control. In a biomedical field, for example, it may take 2-3 years before you have collected enough samples or you might have to wait for a clinical trial to finish before the data can be unblinded etc. It can take a year or more to perfect an assay, the list is endless! ;o)

If you are fortunate, hard working, intelligent, very good at research and a bit more fortunate, it might be possible to finish work for your PhD in two years. It is however quite unlikely that the university would let you finish - it gives a bad impression if universities give out PhDs for only two years work (and they'll be keen to get a third year of fees from you). Also no matter how well you've done or hard you've worked you'll still only have 2 years research experience instead of three. In practice therefore you would spend the third year pretending to still be working and handing your already finished thesis to your supervisor 1 chapter at a time (as if they're only just completed). In the mean time you could have a cheap off season self catering holiday in Malta for 6 months or get some work to pay off your debts.

In reality most students do not submit their thesis within three years, let alone two. By all means make it an aim, but don't plan your finances around it.