Had mine replaced with plastic guttering 18 months ago and very happy with end result.
The company used their own scaffolding system for access; cut away the old concrete gutters almost flush with the brickwork, using a diamond cutter. Timber plates were fitted to the remaining concrete blocks and then these were covered with uPVC facias and soffits. Brackets for the guttering is attached to the facia. The first row of tiles is pushed back and new felt positioned if necessary, plus an extra felt to guide rainwater into the new gutter is added. New gutters are attached, together with replacement downpipes.
The company arranged for a skip to be delivered, filled with the old concrete gutters and pieces of left over products/rubbish, then taken away on their last day of work (included in the cost).
The concrete cutting process was a bit noisy, but I was confined to bed at the time having not long been released from hospital.
Total time taken was about 1 week (it would have been a day shorter had the guys not accidentally locked their vehicle with the keys and contact mobile phone inside at their overnight hotel !).
Cost was about £4,000 for approximately 30 metres of guttering, being the front, back and one side elevation of the house.
The company we used was:
http://www.weatherguardhomeimprovements.com
who I would recommend, although there are quite a few companies that undertake this work, just google 'Finlock Gutter Replacement'
Like The Builder, I haven't heard of a ventilation system for gutters.
These gutters came about, before the advent of plastic guttering, due to the shortage of metal gutters together with the need to rapidly build houses after the 2nd world war. Very useful for leaning ladders against when accessing the roof or relining the gutters, but a pain when they leaked or looked as if the facing was going to drop off at any time.