draining complete CH system

[Nearlyman]

Have noted the post re "draining an open vented system", which unfortunately wasn't right for my situation.
I "own" a detached 7-yearold 4-bed property on a medium sized, fairly 'upmarket', modern development so would have expected at least similar standard of plumbing etc etc to that of my previous, [owned from new] and similarly valued, albeit, roomier 10-year old 3-bed property. However, I find there are - unlike the previous house - no drain cocks at all on [b]any[/b] of the radiators: my previous, house, on a smaller development had them on every rad. I had no trouble draining down this system,(in order to relaocate a radiator), but in this [newer, more(?) modern] house there is no 'convenient' connection I can make in order to drain down the system. I want to fit thermostatic valves to all but two of my 10 rads so I will, of necessity, need to drain down the entire system.......... won't I?
My CH is a sealed system and is serviced by a Saunier Duval combi gas boiler, which contains a small reservoir as a back up for full flow hot water and which works very well: I do not really want to drain down the boiler (the reservoir) as I feel it will not only make the task more protracted but is also, as I see it, unnecessary.
I do not, however, know where the isolating valves are on this boiler to prevent its draining down.:=(
What would be the best, most effective approach to "getting round" the draining issue? And, with the existing rad valves sitting very close to the regular steel panel rads will I indeed have enough room to actually fit thermo-valves?

[plumbbob]

Check that there or no drain valves outside the property. Sometimes on large systems plumbers fit taps outside to make draining easier.

I am not familiar with that make of boiler but there are several on the market that have the small reservoir. Actually, they don't contain any extra water in the primary circuit, the store is pre-heated DHW for draw off at the sink so there is little advantage in trying to avoid draining the boiler. In fact, chances are as soon as you attempt to refill the system the water will be temporarily replaced with air anyway.