The heating drawings you have were not done by the architect on the original plans. The developer has had this done himself probably because some rep from Quinn approached him and offered. The designs are done for free on the premise of specing their materials.
All the boiler and radiator manu's will do a design if they are promised the sales.
90% of builders don't do this and it is left to the contractors who may or more often not approach the manu's. Any plumber worth his salt should be able to design the system themselves.
If you are left to spec the system you can then play the various manu's against each other for bigger rebates which is more money in your pocket.
The risk with this is you have a bigger outlay and potentially more to loose if (should be when) they go t..ts up.
This is one of the main things to watch for when doing one offs. The plumber is usually one of the last trades to completely finish on a new build by which time money is getting tight as justlead says. A new dining table or fancy car for the new drive is often more important to most than paying the plumber at the end.
That is why it is important to load the earlier stage payments to leave as little as you can lying out for completion. Try to keep the last payment to below £500 if you can.
I have said many times i would never work for builders again but relented on a program 4 years ago only as i knew the site manager who near pleaded with me to do the work.
On bigger jobs you put in a monthly valuation for the work done and it is paid the following month. (Sometimes they want a 5% builders discount and a 2.5% retainer. If they do build it into your price if you can, as most likely you will never see the retainer.)
Things went well on that one for around a year and a half then the houses stopped selling and within 2 months the banks were in and i was down 16K.
The builders were full of the usual apologies but that doesn't pay the bills. Around a year later i also got the obligatory letter from the receivers stating the obvious.
I will never work for builders again. The money can be good as can the continuity of work but the risks are always there.
If i had been paid all the money i was due from builders over the years i could have bought a big villa in Spain complete with the swimming pool for cash.
Thinking about it though i would have probably spent it on the usual rubbish