Gas Fire Chimney Flu

I have just fitted a 19th centenary cast iron fire surround and have had an ornamental ‘gallery’ gas fire installed complete with gas approved chimney cowl. Had the chimney swept beforehand but there still soot dropping on the fire when it rains which I’m sure isn’t doing the fire any good.

A friend of mine said the only way of stopping this would be to fit a flu which leads me to my question:

1. How much would a flu cost to be fitted by a professional?
2. Is it hard to do or could a DIY’er like myself carry out the procedure?
3. Is it something that can be done now the fire has actually been fitted?

Appreciate any comments.

Most DEF require a class 1 flue.

What type of Cowl has been fitted.

Thank for your reply.

It has a standard Colt high top gas – does this help?

Also what size flue do I need to get – does it depend on size of chimney or fire installed?

DGF usually 7 inch liner req expensive and an absolute sw#ne to get down if it will at all .