I have just been "Googling" and have come to the conclusion that it must be a Synodontis decorus.

These fish seem to be very shy ....they always seem to be hiding away somewhere ....can anyone give me any ideas how to help these fish be less shy ? as I hardly ever see them

It does look like a synodontis decorus.
I have one and they are quite shy.Firstly you have to take onto account that they're nocturnal. Provide plenty of hiding places ,rocks tubes wood etc and once they become more confident they will come out to feed.
They may be shy because of other tank members, so what are you keeping them with.
Syno decorus also get pretty big upto 9-10 inches which doesn't look much on a ruler but when you turn it into fish is pretty big
HTH

They share the tank with 1 black mollie,1 comet, 3 male guppies,3 cory Juliis, 1 cory panda and 1 mickey mouse platy

You've got a bit of a mix going on there,
None of the fish you've got should really bother the synos though could I suggest you get the comet moved as it shouldn't really be kept in with tropical fish and will also grow quite large.
The other fish will also be at risk as the decorus grow, as the will eat smaller fish.
What size tank are you keeping them in and with what filtration?
Could I suggest you go back to the shop you bought them from to confirm what they are.

the tank is a 90 litre ...22 x 16 x 16

I think you need to go and confirm what this fish is as a 22" tank is not big enough as it grows, unfortunately height is not too important.
If it is a decorus I would recommend that you either remove it soon or best idea, get a bigger tank

If it is what you say it might be ...how long do I have before it grows to a size that it should be re homed ?
It is still a juvenile at the moment.

Youd really need to do it sooner rather than later so you avoid stunting it...

Must agree with peediepixie. Sooner rather than later is better.
I don't know how fast they grow as mine was quite large when I bought it.