If your mortgage is with the EBS and not fixed you can credit your mortgage account with the lump sum. It will reduce the interest charged but you can take all or part of it back out as you wish so if you think it would be better elsewhere it is very easy to withdraw it.
Are you 100% sure about this in all cases? This suggests that
EBS are operating offset/current account mortgages by default. I would be surprised if this was the case and that the customer did not need to make it clear what s/he wanted done with overpayments (e.g. keep them as a credit on the account, same but offset them against capital outstanding or actually pay them off capital thereby preventing you getting the money back without a top-up). As always I feel that it's important to put your instructions in writing and get confirmation in writing from the financial institution.