This is why I think we really need buffs to militia (esp vs champs) and buffs on high-level buildings (requiring city level 3-4, maybe some buildings such as schools should get a bonus per city level?)

Thanks for all the responses. 

Tuidjy; I developed my own sovereign based on your suggestions and it felt almost like cheating. Running around the map killing almost everything and fast levelling.  

The problem I was having on expert if I had a couple of cities was that the AI's would attack me (due to weakness, proximity etc) with stacks that I could not defend against. On hard/hard I could survive the occasional AI dow with a couple of cities but on expert the dow's were almost a certainty and I couldn't survive. 

With a modified sovereign I think I might be able to give it a shot though even with a couple of cities.

It might be as I am on crownded maps (medium size with 5 AI's) forcing the AI to attack early but I was certainly not fine with 2 cities for anything other than a very short time.

No doubt this is a failing in my gameplay and not your advice, but the reason for my first post was due to my understanding that it was the expansion constraints losing me the game.

After further thought I am guessing that using the preset sovereigns and/or crowded map selection probably were just as guilty.