We have recently upgraded the batteries with the old lithium-ion batteries from our Citroen C1 Evie electric car. Even second hand these batteries are much better than the old lead acid ones.
We built our first Solar PV system in 2011 so prices quoted are not current. We used:
12 100AH deep cycle gel batteries (at £45 each from EBay) total £540
10 160 Watt Solar panels (From a contact found on EBay) cost £4000
A100C Import /Export meter (OFGEM approved) Cost £42 from Universal Meter Services
Morningstar charge controller SR740TL with remote digital display - (to charge the batteries from the solar panels) from http://www.barden-uk.com cost £246.
APC su3000rmxli3u rack mount UPS (un-interruptible power supply) from EBay to use as an inverter (a cheap way of changing the battery DC to 'mains' 240volt AC cost £187 (including delivery) highly recommended seller email: email@example.com
I am using galvanised steel C studs to fix the panels to the roof saving about £400 on using aluminium section. The brackets are table leg fixing brackets from a hardware store
I am a little disappointed in the efficiency of the battery based system which seems to be about 60%, Systems which feed into the mains should be much more efficient (over 90% I believe)