Who is offering it? If it’s a deal with the bike like px sometimes offer then it might be worth it. If it’s just someone with different size stems then £50 might be a bit overpriced.
You get what you pay for.
A friend of mine used the bloke at Manchester bike arena, wasn’t cheap but he also does bike fits for a lot of pros. He’s got a list of specific measurements that he sets all his bikes up at. He says it’s better money than spending it on a better set of wheels. His knee problem was also fixed over night.
Another bloke I know who is retired had one done recently. He’d been using the data from over 20 years ago but he’s shrunk a bit and it was all wrong. Again, his injuries have started to clear up.
Depending on the bike fit and what they are offering it might be worth putting that £50 towards a better one.