Hi there, we have had the same problem, it Idles, sputters, stops and starts, but we finally managed to finish cutting the grass.
Hubby did strip it down, cleaned plugs, changed fuel, and tried to fix it, but it still did the same.
In the end we bought another one at the weekend, another Mountfield, as this one had lasted us for a good few years, and were happy with its performance, as we have had others, but still prefer Mountfield.
Like Trazor says, unless you know what your doing, or you can get someone to service it.
Hubby said he would rather buy another one, as he wasnt too sure if it still would have worked after whoever looked at it.
There is companies that buy these mowers, and sell them as reconditioned.
If your thinking of buying another Mountfield like we did, then the cheapest place found was B&Q wharehouse, they had a special offer on, as we trawled everywhere, and on the net, and B&Q had the best deal.
I cant remember the model number, but it was a bigger horsepower to the orginal one, bigger cutting thingy, and a key start, the old one had the cord start where you had to pull it.
It worked out we got for approx £120 less than the original price.( according to the display board)