Diesel engine turns over slowly but won't start?

I have a 1995 ford fiesta lx diesel, tried to start it this morning and it won't start. The dashboard lights came on and it turned over slowly - got slower and slower on further attempts.
Four weeks ago husband flattened the battery by leaving the air con running but the AA charged it up again, so wondering if the battery is weak or if it's likely to be something else? Haven't got Home Start and don't want to call out mechanic if it's just the battery. Also is it very difficult to change the battery yourself?
Sorry that's about three questions in one!

It does sound like your battery is not fully charged and each try to start is weaker. Also check the belt on the pulley to alterator is tight.

It's more than likely going to be a glow plug issue I would have thought.

However, rather than call out mechanics, first try a set of jump leads (hooked up to another running car) leave them attached for 5 mins then, turn your ignition on, wait for lights to come on, then glow plug lights should go off( yellow squiggle) then try and start the car.

Repeat this process revving the donor car for the jump leads if it fails first time… Make sure you connect the leads correctly!

Sonds like the battery may be weak/failing would suggest checking the battery and alternator. Check the alternator belt to see if it's there and or tight, get a jump from a friend or neighbor and take it to a parts place and have them check the battery if you need one get one. Not sure about your car about how hard it's to replace the battery as I'm not familiar with you model

Your battery is weak. Once a battery goes completely dead, it is severely weakened. An old battery will be damaged, if allowed to go flat. It's not hard to change, if you have the proper tools (wrenches, sockets, extensions). It will weigh 30-40 lbs. Take the key out of the ignition, and turn everything off. Disconnect the negative cable 1st, and reconnect it last on the new battery. Was your hands good afterwards; because battery acid will burn the skin, and eat through cloths.

Thanks everyone, sorry should have said it had new glow plugs about 6 months ago so hopefully it's not the glow plugs

Well, it sounds to me like your battery has had its day. Get a jump from a helpful passerby and get going - straight to the auto parts store. Have them load test your battery.

Once it's come out that your battery is toast, get a new one.

I don't see how Hubby leaving the aircon on could have anything to do with the battery dying unless the fan keeps on even after the car turns off (and don't be misled by a fan running under the bonnet after the car turns off, several cars do that).

If this is the battery that came with the car, a 1995 battery, I'm frankly amazed that it still works and it is ENTIRELY likely that the battery is done. Even if you had it replaced just a few years ago, it's not that big a deal.