I own several XLA flats in west london

time to change the combi in a 60 sq meter flat
Contains 5 rads-incl bathroom

After much problems with the potterton
Engineer advises its important to obtain correct boiler for a 5 rd system

Seems many boilers too powerful (set for 10 rds min) and problems caused -eg overheating

Can anyone advise most suitable type of combi-incl cheapest to buy and maintain!
many thanks

David Noble