Lots of tradesmen, one person to project manage?

I think you should go with one company who will carry out all works, that way it will save you having to select all trades, and trying to coordinate them

Both options have own profit. When you organise separate traders to do different jobs you should save some money IF you can manage time schedule and cooperating between them. Also important is to have in mind works, which usually, labourer take care about. Like sanding walls after plastering - before painting for example. Someone who estimate painting usually will not repair any cracks, or do not sand any left overs for you. This can rise the running cost. The easiest way is to go with company, where you already have person who manage everything for you. It will be more expensive - but ONLY if you have skill and knowledge to take care about job. Other way? Hiring company will save you stress, time and money after all.