Most reviews tell us about battery life in new mobiles. But from my experience, by 18 months into a 2 year contract, with average use you are lucky to get a day's use out of it.
I am shortly to get a sim free mobile to replace a standard 2 year contract, and will source a monthly sim only deal, with the intention of using the phone for 18 months.
But which mobiles hold their charge best in the long term?
(I am thinking of spending £150-£250 on the phone.)
Thanks.

You could always buy a new battery. Cheap copies on eBay for under £10 or genuine ones for about £20.

That's true, but some are sealed in?

That just means that you may not be able to replace it. Many phone repair places can though.

It's also possible (more likely) that your usage has changed, rather than there being any appreciable difference in battery capacity at 18 months.

If your phone is out of warranty then you can fix it yourself, instead of taking it to a phone repair shop either which way your warranty will be voided.
For most - but not all phones replacing a sealed in battery is relatively simple.
And its also surprisingly easy to fix a multitude of hardware damage problems yourself.

And if your thinking about buying a phone then factor in its repairability before you do - how easy is it to repair a broken screen? how easy is it to repair a broken charge port/headphone jack and how easy is it to replace a battery?

this website will potentialy save you lots of money in hardware fixes
https://www.ifixit.com/
And if you have an android phone thats out of warranty, getting old and slow then this website and its forums will show you a way to update the software and in turn will give an old phone a new lease of life http://www.xda-developers.com/

Most Android phones are replaceable easily enough. Even Apple phones you can do it yourself.

DO NOT BUY A CHEAP BATTERY. They will be shite and potentially dangerous.

Batteries should easily last the 2 years though if you look after them. Making sure they don't drop below 20% change helps and try to ensure you don't fully discharge then as well as that knacks them. In an ideal world you would keep the charge between 20% and 80%.

Sellers do not want you to know, They can produce batteries with 1000 charges but obviously that means less replacement sales.

Current best bet is to buy a phone with a removable battery. And replacement cost of a quality should be near £10.
The only good news is that slower charge increases lifespan, and that the bigger the battery the more gentle the charge should/can be.

What miffs me is that I have found no app to let me cut off the total charge at 80% for the times when I know 80% lasts me the day. An 80% only charge will likely double lifespan too, the more critical charge stage being from 60% charge upwards.