I have a 4 1/2 month old chocolate lab called Molly. She is lovely but has started chasing her tail and is driving herself crazy. She was wormed about 3 weeks ago and has been treated for fleas. At the moment, we try to distract her with toys etc but she still goes back to the tail chasing. I would be grateful for any advice. I am at home with her so she is rarely left on her own.

Hi Audrey

This is more behavioural than worms or fleas if she is doing it frantically. Some dogs will react with this obsessive compulsive behaviour if they have undergone some kind of trauma like a change of home which could be the case here as she is so young.
Something is making her anxious and stressed and you need to break this cycle. Be calm around her. You are doing the right thing by playing with her and distracting her with toys... but as soon as you stop then she is reverting to this frantic tail chasing. I feel this is another big advantage of the crate. Play with her and tire her out with toys then pop her in the crate where she won't be able to chase her tail. You could also keep her on a lead indoors with perhaps a little headcollar on so you can stop her straight away. This won't be too difficult if you are home all day.
What you have to be careful of is that she doesn't do any damage to her tail. It may be necessary for her to wear a muzzle if she is actually injuring herself.
Hopefully she'll stop soon.