Hi Gabby.
F05 indicates a problem with the pressure switch --the system that determines correct water level in the drum. Its a round item about 3 inches across with several wires connected, it sits under the top cover. Its an air operated switch which tells the machine water has reached the correct level. Disconnect the thin rubber pipe from the switch and blow down through it to clear any blockage. Switch itself may be faulty but its more likely the pipe is blocked.
Also check the filter - generally located behind a panel at the foot of the machine.Some are obvious like little square hatches and some are hidden behind the trim panel at the foot of the machine. Once panel is removed the filter should be visible. Its generally a circular plastic fitting which unscrews in an anticlockwise direction. Some hotpoint, indesit and ariston filters are located in the big black sump hose (the one that leaves the bottom of the outer drum and connects to the pump). Remove the back cover, locate hose, remove the tensioner clip by squeezing the tabs together, remove the hose, take out filter and clean.
Check out www.2ndwave.co.uk/manual.html
dannyboy, October 2008