Hi James it will probably only last the morning on first cleans, however in the future 250L will do you a short day.
Ways that you can prolong the tank are
1, Trad the downstairs until you have all the first cleans out the way, this can help on the switch over as well with some fussy customers.
2, Get a water heater, you will use less hot water than cold water and also it's nicer on first cleans. I noticed you are based in Essex, there's a guy in Basildon called Craig that fits them, he does a good job or you can purchase them off the internet cheaply and DIY.
Hope you enjoy the system, don't get to stressed with it.
Simon.