Bottom oven is not working even though the top one is 720EF model

Look for any burned or discoloration on the bottom bake element. Did you try the broil on the lower oven? If neither is working you may have a board or touchpad issue.

Try adjusting the hinges also check magnetic seals on door itself as well.

It looks like the clock has failed. Try leaving the power off for at least 10 mins, turn it back on at the main switch and then try it. If it still doesn't work Espares can get you a new one. Incidentally, if there is power to the oven, the grill will work even if the clock is not set.

Both the top and bottom oven will only work if a time (any time) has been set on the clock. Setting the time operates a contact to switch the power through. The only exception is the grill. This will work even if the time has not been set. So if the oven controls/lights and heating work, to me it means that the clock/timer is half working and the only thing that isn't is the display.
I haven't had the clock/timer to bits but a fellow blogger has said that in some timers there is an electrolytic capacitor that controls the display. When this fails as they are renowned for doing, the dispaly mal functions. If there is one then Maplin have some - VHR41W/43W and DT64U depending on value etc. Let us know how you get on.
If you need a new clock/timer they are around £80 from espares.

You have to remove the entire oven unit from the kitchen carcass. Once you have done this, you will need to take off the top and rear panels to gain access to the items you want.
There are normally screws holding the oven in place and a pair of these may be seen once the top oven door is down. They are located near the top of the opening on either side.

No, there is no slow cook. The bottom oven works as a normal fan assisted oven and the top oven relies on elements concealed in the floor. The top oven takes a long time to warm up.

When you say the main fan, do you mean the background cooling fan or do you mean the circulating fan in the bottom oven?
Do you have the handbook for this oven as the wiring diagrams are included in it. If you haven't, you can take a copy from my BT Digital Vault - all I need is an email address to register you.
Looking at the diagram, the power for the cooling fan comes via the timer board. If, as you say, the fan doesn't work at all, the timer board could have gone faulty.

FAULTY ELEMENT OR THERMAL SWITCH. HAVE IT CHECKED OUT BY AN ELECTRICIAN.

there is a safety stat/fuse on top of the oven exterior i think you will find that it also has blown. Make sure you have isolated the appliance at the fuse board first though, Good Luck

According to the circuit diagram on page 31 of the handbook, power for the ovens passes through the clock. The power feed is direct for the actual clock but via a seperate Thermal Cutout for both the top and bottom ovens. I think it is highly unlikely that both Thermal Cutouts have failed at the same time. I would try turning the power off totally to the oven for a while and then turning it back on, set the time and ensure the oven is in manual mode. As the top and bottom ovens have seperate controls after the Programmer and both are not working, it has to be the programmer that is not extending the power through to the rest of the oven.