An old system as you describe and only one tank in the loft-space? could suggest a primatic cylinder, bit awkward to work on for a DIYer, I would get someone in to have a look and confirm the problem for you. best of luck with it.
If you can spare a day away from work a company called Kamco based very near to St Albans have a one day course on how to use their power flushing machine. google their web-site for available days.
From your description, it sounds like you have a plume management kit fitted to your flue, from what your saying it suggests that some of the condensation that forms in the flue system is dripping onto your patio.
It really should be contained within the flue system and be managed by condense...
I don't want to contradict muggles, but if your having a brand new installation, you could size the radiators to run at 60c, allowing your new boiler to stay in condensing mode longer making it more economical.
Good luck with your build.
From what you have said it would seem that every radiator downstairs has it own drain valve, its a bit of a pain but just connect your hose to each radiator one at a time and drain them off.
No need to shut down the flow valves, connect your hose nice and securely and let it go.
Running your...
With the upmost respect, if your are struggling to find out which of the pipes is the flow, freezing pipe-work on the second floor of a tower block might not be the best idea you have every had. At least there is only one or two flats below you that you could possibly flood.
Are you Polish? just like to say how good your written English is for some one who has only been in the country for such a short time.
Many congratulations on your visa application.
I believe Tamz to be correct on this matter, 32" the average inside leg measurement, derived during victorian times so a Gentleman could wash his willy in a public loo should the need arise.