Thanks for your replies. I'm going for one that's advertised at Screwfix. Mine too is for the allotment. I'll probably only use it a couple of times a year as current thinking is to leave grass pretty long to give wildlife a habitat.
Don't forget that a petrol strimmer will need petrol/oil mix. That's a fiddly dirty job but results in a powerful strimmer. You have to mix petrol and oil in the right proportions, read the instruction book twice a year (because you have forgotten it from last time), extract and clean the spark plug, clean the air filter, and so it goes on.
For 2 strims a year, I'd advise a rechargeable battery model.