The house I've bought has a Lamona HJA4560 double oven but I do not have an instruction book. I don't know how to re-set the time function and I haven't been able to find an instruction manual online.

Most likely it is a control panel failure

What do you mean by 'Casing'? the only wires coming off should be display wires to a screen or digital clock. If your screen has failed or you no longer have a clock display, this is where your wires came from.

Try this link it might help
http://www.docstoc.com/docs/71826497/Lamona-Double-Fan-Oven---HJA4560-Lamona-Built-Under-Double-Fan-Oven

Firstly, make sure the actual time is correct. Set the oven (if it's a double oven the clock function will only work on the main oven) to the setting and temp required. The first timer setting you put in is the cooking duration (not the start time) ie if it's a casserole that you want on low for a couple of hours, adjust the first setting to 2:00, when you press for the next setting you will see the the timer has already added 2hrs to the current time, adjust this to the time you want the item to finish (eg, You want it ready for 6pm, adjust to 18:00). Don't press anything else, give it a second or two and the oven will switch off. The oven will switch on 2hrs (or whatever time you have set) before the end time. Whilst the oven is waiting to switch back on, on some models the cooling fan will run continually, this is normal. If you are a little slow enter the times the clock will time ut and return to the actual time.
Hope this helps

I hope you have found already solution for this... Anyway this may help others: press buttons 1 and 2 together and release. Then use jog wheel to adjust time. Wait 5 seconds and you're done. Gorenje manual sucks, no info about oven controls. I managed even to set on oven child lock (key symbol), only way to take it off was to remove fuses from electric board of our house!

Hi, I understand your frustration, the manual supplied is next to useless! The 'obvious' settings appear NOT to work (as the grill remains on full heat setting all the time). The only way we've managed is to lower the food away from the grill..... not ideal I know - but also we don't use it for toast.
I suppose Hoover would be the next best port of call for assistance, oh for a proper manual, not a horrible generic one that attmpts to cover every model they produce - AND FAILS.
This is purchasers main complaint about buying Hoover ovens it seems.
I'll come back to see if anyone can supply a solution to our problem.... and hope it's not a fault in manufacture.

Hi, hold down the minute and cook button and at the same time hit either the plus or 1 button to set a time and then oven will work. each time the power is off this needs to be done. hope this helps

get manual here

http://rapidshare.com/files/369550394/BellingFormulaMultifunctionDoubleOven.pdf.html

good for 60 days only