dont expect miracles with a tiny solar panel
on my previous house i had 17 solar panels on my roof ( nearly 4kw's)
during the winter months i could create 60 wattts of power from them
so i doubt the solar panel from halfords would make any noticable difference to
the state of charge in your battery, i've also heard that when using
a solar panel you should use a solar controller otherwise during darkness
the solar panel can take power from the battery and start to drain it.
if your using an OBD2 adapter again dont leave it plugged in all of the time
as this will drain power from the 12v battery and help to flatten it.