Just to be transparent, it's hard to give objective advice here unfortunately. Firms will likely differ widely on this point, depending on whether they are large, small, high street, international, UK headquartered, US headquartered, recruiting a large or small trainee intake, and so on. 

So, with this in mind, here are some general thoughts to consider when thinking about the question "do A-level grades matter to law firms?".

Minimum A-level requirements

Some firms might include minimum A-level requirements. With some firms receiving thousands of applications for dozens of roles, this can be an easy way to narrow down the number of applications that need to be reviewed (psychometric test benchmarks can also play a similar role).

Some of these firms may however pay close attention to extenuating circumstances and frequently make exceptions, whilst others may not be quite so flexible. 

Are extenuating circumstances relevant?

Hannah Salton - ex-Magic Circle recruiter and author of Graduate Careers Uncovered - agrees, emphasising that "law firms really do pay attention to mitigating circumstances, so if they apply to you, make sure you share them in a clear, objective way- specifying when and how your academic performance was affected".

If you're unsure whether your circumstances are relevant...

• Perhaps check with firms what kind of extenuating requirements might be sufficient to justify grades that don't meet their minimum benchmark. The nature of extenuating circumstances can vary significantly, so it's worth trying to find out whether your circumstances may be sufficient to warrant the firm deviating from their default requirements.
• Do however be realistic. If you have only just missed a firm's minimum requirement (e.g. you secured A, B, B rather than a specific requirement of A, A, B) it's probably more likely that the firm will make exceptions than if you secured C, C, D.

If my exam results have improved, can this help my application?

Some firms might value continued improvement over a number of years, rather than - for example - strong A-level grades followed by average university grades. Improvement indicates progress, and - in my opinion at least - your more recent grades are likely to be a more accurate reflection of your current capabilities.

If you missed a minimum A-level requirement, but then smashed your first and second year university exams, it's probably worth messaging the firm to see whether they'll still look at your application (even if you don't have extenuating circumstances to explain the lower A-level grades).

If you also have a great base of professional and extra-curricular experience, there's a chance this will help to balance out lower grades in the eyes of prospective employers.

What if law firms don't have minimum A-level requirements?

Some firms don't include minimum A-level requirements. In many cases, this hopefully means that the focus is only on what you've achieved post-A-levels. However, there's of course a chance that firms have publicly removed A-level requirements, but in reality still take A-level grades into account when making offers (although if firms don't even require you to include A-level grades as part of your application, then this shouldn't be a concern).

As a caveat, some firms place a real onus on university rankings (whether or not they publicly admit this), which can indirectly introduce A-level bias, even where no minimum A-level requirement exists.

Given that higher ranked universities - as a generalisation - tend to have higher A-level grade entry requirements, this means that law firm preferences for higher ranked universities can indirectly mean a preference for candidates with higher A-level grades.

What are my options if my A-levels aren't good enough?

Peter Watson - ex-head hunter and founder of Watson’s Daily - points out that if you’re not able to secure a top role because of grades, this doesn’t mean you’re stuck forever. 

Perhaps start at a lower ranked firm with a more flexible recruitment outlook, use this time to demonstrate real learning and development, then apply for a top firm again further down the line. Hopefully by then your A-levels will be fair less relevant, as the focus for experienced hires tends to be much more on the work they’ve done.

That being said, if you're on the outside, it's ultimately hard to know for sure, so this is just me theorising! I personally don't think someone should be judged based on exams they took many years ago, especially if they have since demonstrated strong academic prowess. 

But I'm on the outside looking in...