safety concern with new insert and liner.

I have decided that my current setup is not safe to burn. I have a Fisher insert in a fireplace without any connections to ceramic/brick chimney. Last year we had at least 2 small chimney fires with a lot of creosote build up.
I am planning to put in a Regency insert with a full flex liner. The question I have is that the chimney has a build up of the shiney creosote. If this creosote is not removed prior to the new liner could it get to a high enough temperature to spontaniously combust in the airspace of the chimney.
Thank you for any help.

there was quite a bit of the fluffy soft stuff that I can brush out easily. I have removed all of that. The shiney stuff that is hard on the surface is just a thin layer. I have not truely measured but I would guess that it is no more than 1/8 inch thick.

Its common practice to do a complete sweep before installing a liner. My installer did this before installing my Regency and liner

Get rid of all that shiny stuff. That's more than likely 3rd degree. And yes it can/will ignite from the temp of the liner. Karen Duke told us a horror story of a masonry chimney actually exploding due to this. Get it cleaned, completely. If it's 3rd degree the sweep won't be able to remove it in one trip. He'll come sweep then apply cre-away (or you can go get a bottle and apply yourself) this stuff really works well. Spray it in the chimney then if you can have a fire or two. If not it will still work but it does work a little better with a fire. It will "dry out" the creosote and convert it to a brushable form.

My sweep roto cleaned our fireplace flue with a spinning chain to get out as much glazing as possible before we relined for our insert install. Seemed to work pretty well and was only an additional $30.