Thanks all. Lots of good an thoughtful input which might be useful to others as well.
I was kind of hoping that a few people would say 'had a cordless, bit rubbish' which would have made my decision easier. It seems like battery is viable, but a careful look at reviews, run time etc. and the general state of my grass (my hayter is 4 wheel, not roller, much as I would like nice stripes nearby trees etc make the ground uneven) I think I would be on the margin for cutting in one go without a second battery. I like the look of Makita but current price with 2 sets (i.e. 4 - they use 18v in series for 36v) is fearsome. I'm sure, as some have said, that the margin is in the batteries - bare shell mowers are relatively cheap. Perfect if I already had big battery tools but I don't - my drills etc are 18v Bosch.
Looking back, mowers seem to last me 10-12 years so I need to think about what I will be wanting when I am 80 (if I am sufficiently compos mentis to even remember I have a lawn to cut). Realistically I am thinking about 5 years ahead and will worry after that. Corded electric just wouldn't work here - too much to get round and it would drive me mad.
I'm currently thinking about going down a size for lightness and ease, adds 10 mins to my cutting time perhaps but I have more time now, but sticking with petrol for the 'ooomph' to get through the tough bits - some areas get left for a few weeks because of spring bulbs. In 5 years it may be that almost everyone is using battery and the market has changed.
One of the problems is information overload. Makita alone have 21 variants of the same range - then there is AlKo, Cobra, Hayter, etc etc. Each has its own arcane numbering and naming scheme. When I was young, people had (and defended to the death) a choice of Webb or Atco, push or if you were posh petrol. Now there are hundreds. Relative to earnings they were very expensive. Now we have too many choices.
My plan is to stop thinking about it until Monday when I will decide and cast the dice.
Thanks again, a rich source of information.
Or (and hopefully not) decisivness will fail me, I will buy a fibreglass kit, bodge the holes in the deck, adjust the stick wedging the air intake open and file it all under "too difficult" until it breaks again. My wife grumbles that I am always fixing stuff long after it is sensible. Barbecues aren't designed to last 20 years....yes but look, if I bend a piece of aluminium and put it in here it will hold the burners up for another year or two ..... and there's nothing wrong with the oven door now I've drilled it to fit clips to stop the glass falling out..... etc.