So, immediately get on the horn with your lawyer and have your lawyer on his letterhead send them a letter and if the response is not to your liking sue the bastards. In the meantime set up another account with another bank and make sure all invoices that are pending but not yet paid are paid into that account and not into the HSBC account. Then shift over all the monthly payments to the new account as funds appear.
Lawyers are expensive, and bank accounts have complicated T&Cs which basically say "We can do what we like, and fuck you." You'll spend a fortune paying someone to argue some or all of the agreement is unenforceable. I would suggest trying to get in contact with other businesses in the same boat. The UK doesn't have a formal class action suit mechanism, but there's nothing to stop you pooling your resources in a join…
Actually, it does.
The Consumer Rights Act 2015 created one. See Schedule 8[0] (which amends the The Competition Act 1998[1], but legislation.gov.uk hasn't updated that yet).
However, there has only been one case, so far,