Could you tell me if the Easi-Heat I 25c could be fitted in place of an Easi-Heat Plus 25ci, without major pipework changes. And Would it fit on existing bracket ?

The order of the connections on the Easi-Heat Plus was Heating Return, Heating Flow, Gas, Hot Water Out, Cold Water In.
The order of the connections on the Easi-Heat I is Heating Flow, Hot Water Out, Gas, Cold Water In, Heating Return, so there would be a little bit of pipe work change.

Also the Easi-Heat I 25c is slightly smaller so may not fit on the same bracket as the Easi-Heat Plus.