We had ours done by a solicitor. My wife is a law lecturer, and she thought that the reliable indemnity insurance and the way that they are regulated & trained, unlike will writers, meant that it was worth paying slightly more. It wasn’t super expensive either.
It possibly depends if you have anything complicated (like you want to specify particular arrangements for kids in the event of both parents dying) or out of the ordinary, but she is the expert and she thought it was worth paying slightly more.
She’d also seen one written by an online will writer, following the instructions of someone well off who had quite complex intentions about a wide variety of assets, and it was complete unclear gobbledegook, along with including factual innacuracies, all of which would probably lead to years in court if anyone decided to challenge it on their death.
More on this:
http://www.moneywise.co.uk/investing/financial-goals/will-writing-scams-to-avoid