the part costs about £100 in your friendly local plumbers merchant
It is IMO very worthwhile in an older system where the water is black or there is black sediment.
If you can do basic plumbing you can fit it (allowing for draining down, refilling and bleeding) in an afternoon. It did one on my old mum's house during the summer.
I am not a plumber.
It is useful for collecting black sediment and will probably prevent a blockage occuring later if this sediment builds up.
The drain down will remove muck which (unlike the black sediment) is not trapped by the magnet.
For best results you can give the system a chemical clean as well (this is very easy if you have a vented system with a small tank in the loft and you know where the drain-cock is).
When refilling, you must use a corrosion inhibitor t prevent further corrosion.
p.s. ask round friends and neighbours to see if thy know a good local plumber. he is most unikely to be as expensive as BG. Be aware that if you want him to also do the drain, flush, clean, refill, replace worn valves, this will add extra time and so cost. If you are having it drained, then it's best to have all the other bits and pieces done at the same time.
I found it took me an afternoon to drain, fit it, fit a new draincock and add cleaning chemical,; then another afternoon a couple of weeks later to drain out and replace a few valves and bleed screws which i'd found needed replacing on my first visit, refill using inhibitor, and bleed again. No doubt a professional would have been quicker.
It would have been even quicker to miss out some of the extra jobs.
