I assume you're studying W36.
The only W36 focus that doesn't require M250 is comms+networking, which requires TM257+TM255.
If you've already completed either of TM257 or TM255:
You should now be able to do the other and complete the diploma without further issue. (You might need to get Student Support to switch which focus you're on.)
If you haven't done either of those then your options are:
(1) Do both, which would obviously cost more/possibly take longer/effectively waste some of your previous credit
(2) Switch to a DipHE Open
Whatever you choose, you should consider whether there's a chance that you'll decide to go for BSc Computing and if so check that your modules are a match for that.