OK, stay on the train and get the 5:35 from Blackfriars (when using public transport in London, always have at least two options). It should get to Blackfriars by 5:30, and I can get from tube to platform there in a minute.
Tube train leaves Victoria. Stops at St James Park. Some people get off, others get on. Train leaves SJP. Stops at Westminster. People get off, including the man with no front teeth standing in front of me, and someone sitting beside me. Others get on.
"Is this your bag?"
Oh no, the person beside me had left her bag - something bland, size of a large handbag.
What would you do
Claim it was yours, so you could have a rummage inside later
Claim it was yours, so you could hand it in at your destination
Pick it up, push through to the door and shout 'Oi! You left your bag!' and give it to someone (with luck, the right person)
Put it on the platform and say 'let's go'
Go 'oh ghod' and get off the train and out of the station quick
Run, screaming 'bomb! bomb!'
Follow the advice on the posters: push the emergency button and get the LU staff, knowing you'll miss your train
Ignore it, and hope everyone else does too
Of course, someone had to pick the 'push the emergency button' option, and so after five minutes faff, I got off this train and onto one going back to Victoria to minimise how late I'd be.