If I understand this correctly, it’s the act of paying into the account that “triggers” the higher rate, not keeping the higher balance, yes?
It all depends on how the conditions of your bank about the E-saver account are worded: Does it say “if the amount of 250 is paid into at least once a month bla-bla”, then your scheme will work and is legal and the bank is not allowed to “wise up to it and stop it.”
If the condition says “if the balance of the account is above 250 a month/ if 250 is paid into the account once a month and not taken out for at least x days / what ever other combinations they can dream up (they have free leave to dream up pretty any weird combination of factors)” - then your scheme won’t work.
As to the bank wising up - if the conditions are favourably to you, then they have to change the conditions first, which will affect not only you, but lots of other customers. Until they do, it’s not illegal - they made they rules, and if they didn’t word them carefully enough, you can certainly use them to your advantage, just as they do to theirs normally.
I know this because we have some banks that offer current accounts free of charges if certain conditions are met, which vary from bank to bank: Bank A (a small private bank) offers no charge if your salary is deposited, Bank B (Hypo…) offers no charge if your balance is on more than half of the month above 1 500 Euros, Bank C (Postbank) used to offer no charge if you transferred 2000 DM (later 1000 Euros) into the account during one month - even if you paid into 50 DM, paid out, paid in, etc. Lots of people used to transfer 2000 DM into the account at the start of the month automatically, and transfer back into the savings account one day later, and it was completly legal. The Postbank took some while to catch onto it that this was not to their advantage and then changed the condition to “keeping 1000 Euros in the account for one month” or similar (at which point, many people closed their account, because that was no longer of interest).
So all kind of conditions are possible and legal and fun to game.