Considering that discounts used to be unheard of on MINI's, £615 seems to be amazing on a current model MINI, especially on a diesel which is the most sought after. Maybe MINI sales aren't as easy as they used to be
Can I assume (as you are asking for delivery) that this is not a local dealer? I'd always prefer to try and get the best price possible out of my local dealer (unless they are rubbish!) as I like to build up a good working relationship with the dealer that is likely to service my car and look after any warranty claims that I may have.