You don't say how many miles are on the engine (maybe I have missed that).
The Parkers mpg estimate is just what Renault promised when the car was new and you can be sure it probably never attained manufacturers promised figures - what car ever does.
You also have to consider you have a 16 year old car and no doubt age will have taken it's toll.
In my experience the older Clio 1.4 was quite nippy for its time.
Fitting a so-called after market air filter may in fact be counter-productive.
I suggest you give the car a good service - change all filters, fill with decent engine
, renew the spark plugs, check the electrical aspects ignition system. And don't forget to check the brakes aren't sticking and tyre pressures are correct.
I think you have to be realistic when driving a 16 year old car which has 16 year old technology and engineering.