Dear Potential Employer:
If I understand the position correctly, what you are saying is that as well as paying me less than my previous role, you now want me to make a further sacrifice while you train me in the ways you want with absolutely no guarantee that my salary will increase after this period / revert to the initially offered level.
While I understand the commercial benefits to the firm of reducing your salary overheads, by offering me the role to begin with you recognised that I bring with me not only a skillset that will be useful to you, but experiences of different working practices that will be complementary and, maybe, better than the ones you have in place. On this basis, I struggle to comprehend why you would believe that the saving of £1500 gross salary over the initial 6 months was justified and am minded, regrettably, to reject the offer.
If you feel able to revert to the initially agreed £30000 then I would, of course, still be happy to proceed on that basis. Perhaps you would let me know within the next 72 hours as I have other career options to consider.
Yours etc .....