How to reduce GCH Quote

MRF

I'm currently trying to get GCH put into my 1 bed flat in South West London. I've had about 5 quotes ranging from 3400 to 5800 (British Gas). The basic quotes I am being given are

Install new boiler (everyone is quoting worcester bosch 28i)
Install rad in lounge (c7000 btu) Bedroom (c5000btu) Kitchen (1500btu) and a towel rail in the bathroom
I have ensured TRV's, lockshields, corrosion agent, controls are all included
The bathroom needs connecting to mains (run of about 4m) so means new pipework to bath and basin as only compression sealed pipes at the moment
Distance from meter to boiler site is about 2m
Fle length is about 1.5m
Pipework will run above skirting (so no lifting of floorboards)
Total pipework length round the flat is about 12m

I have researched supplying materials and if I buy the boiler, rads, TRV seperate then it will cost about £1k). I can't see where all the extra 2.5k is coming from as most of them say its 4 or 5 days work.

Has anyone got any advice on how I can get this down, would hanging the rads myself make much difference? Or have I just been unlucky so far with the quotes as even at a high day rate of £300 there is still only £1200 of labour in this

Any advice would be welcome

MRF