Corsa-B.uk - Vauxhall, Opel and Holden Corsa B Owners Club

After a few months i have realised that im not getting good mpg like i was a while ago... i got on red light the other day so i put £15 worth of petrol in at 134.9p a liter. I have driven 85 miles and im on red light again, it works out at about 33-34 mpg and i used to get in the 40s. could a leaking exhaust contribute to this and also would a full service help? as in: Fuel filter, oil filter, air filter, spark plugs, half a tank of fuel with some redex and some new oil?

Also whats the easiest way to get to the cat on a 1.0L corsa to take it off as i got a new one to be putting on tomorrow so i just need a bit of help!!!