Hello Jamie,
Since you having (occasional) symptoms akin to battery failure, then the first port of call in an investigation is to have the battery checked. Process of elimination.
Either charge the battery overnight, or take it for a 20 mile run, to ensure it should be fully charged, then get the battery efficiency checked - Halfords will do this.
If the battery is found to be suspect - would I buy a replacement from Halfords??
I always buy either Varta or Bosch (same) batteries at competitive prices from on-line quick delivery battery specialist suppliers - not from three worded on line motor factors offering high discounts on inflated ‘retail’ prices!
If the battery proves to be serviceable then let us know, or better still, let us know anyway.
Kind regards,
Gareth.