Reddit reviews Guidelines for the Assessment of General Damages in Personal Injury Cases

Whilst the incident may have left you with more than a bruise, that is not the legal question.

The legal question is, what injury did the breach of duty cause, and was that injury reasonably foreseeable?

A bruise from an incorrectly sent needle, probably.

An increase in the pain medication you were using - unlikely but depends on what the (correct) needle is usually used for and whether that would reasonably be in the contemplation of the company: was it 1) reasonably foreseeable in the circumstances that you were even on pain medication, and 2) what is the cost of that; the pain actually suffered would be part and parcel of the bruise damage.

Anti-depressants: what is the damage for using anti-depressants? Did this wrong needle cause a new psychiatric injury? Eggshell skulls notwithstanding, it seems hardly reasonable to expect that a minor injury from the wrong needle has caused e.g. PTSD in the usual case (maybe if the medication was used to treat needle phobias?).

You say the company has accepted liability? You will need to be very sure as to the context of that liability - have they simply accepted breach of duty? Or have they also accepted the quantum of damages?

And the impacts on you mentally: English law will give little in the way of damages for distress in these circumstances - and by little I mean, usually £0 - unless you can show psychiatric injury. The pain and suffering damages would be subsumed into the award you receive for the bruising.

Realistically, unless there are extenuating circumstances in the relationship between you and the company - your medical history is unlikely to count for much here - then we're only looking at damages for the bruising.

Common law is simply a system of jurisprudence whose main facet is stare decisis. What you need to look at is damages under English law for personal injury - and the main publication on that is the Guidelines, published by the Judicial College, which you can buy here: https://www.amazon.co.uk/Guidelines-Assessment-General-Damages-Personal/dp/0198814526

To put it into perspective - a soft tissue injury that has resulted in bruising, that heals within a week or so, will see you get £550 max. I'm not sure where your injection site is or how extensive the injury was, but that is the kind of thing you're looking at.

£8,000 for wrong medicine depends on what injury they have suffered. For instance, that is similar to an award if I cut your finger off. I don't see minor bruising (unless accompanied by something else), attracting anything more than a low 3 figure award.