Can anyone recommend: A tradesman to fit a cat flap in double glazed door

not to discourage you, but we investigated this possibility some years ago, through a well known local glaziers. It’s not that simple as cutting the DG breaks the seal retaining the gas, usually argon as I understand. It would involve making a complete new DG unit, cut during manufacture to accommodate the cat flap, then removing the existing and fitting the new. With commensurate cost.

We cut a hole through an adjacent wall instead and installed the cat flap there, which gives the result of a small tunnel, but it works.

Oh dear . Yes I had that done in the uk when we were having new double glazed door . Ok thank you I’ll have to think of a plan B then . As we have a a visiting ginger Tom . So it has to be one that only lets her in . 
thank you for replying 

The only ones that really work against visitors are the electronic ones which work on the cat microchip. We used one for years in uk and then here. Unfortunately we now have two semi feral cats, domesticated when it suits them! And we’ve not been able to train them to use it!

Yes that’s what I had in uk . Was hoping to do the same here . Thank you Stephanie I appreciate your help 

We had a similar problem in the UK.  We replaced the bottom panel in the door with a sandwich board of foam and plastic sheet of the same thickness as the glass panel. Then we installed the cat flap into that. It's very easy to cut.

The sandwich board is commonly used in double glazing as a blanking panel where you don't want glass.  In the UK it's easy to find, but I don't know about here.

Thank you . We did the same in uk too . 

Our DG doors are ‘faux’ panels, ie the horizontal bars are within the glass sandwich, thus imitating a four panel door, but in actual fact simply full size sheets.

I know what you mean about 'faux' panels, but if you are taking the whole glass panel out of the door and replacing it with a sandwich board of the same thickness it doesn't really matter.  The double glazed unit will come out in one piece.

Yes, I understand, but it’s a personal thing, we have four sets of French doors in the kitchen and  we wouldn’t really want a wooden panel in the bottom as we would find it visually irritating. And we knocked a hole through the wall instead!