It appears that you are willing to spend approx. £50 a year on strimmers that are useless. Why not spend a bit more and you will have one that will be reliable for many years and wont need the blades/cord replaced every time you use it. I bought the one shown below 4 years ago and I use it every couple of weeks to maintain the paths around 2 allotments (approx. 120 metres). It start's 2nd or 3rd pull and doesn't use a gallon of petrol over the whole season.
http://toolstoday.co.uk/stihl-fs50-c-e-grass-trimmer?gclid=CLbmppbdosUCFTPMtAodChUAwQ
On the long run it is a good investment because of the resale value due to being a recognised brand. People will queue up to buy a second hand Stihl and if it has been cleaned and looked after you will usually get £100 for a 4 year old privately owned model.
The only maintenance I have done with mine is to run it dry at the end of the season and don't store it with fuel in it. At the start of the season put fresh fuel in and off you go.
It's a no brainer .. it will cost you about £50-60 over 4 years for an ultra reliable bit of kit.