On Wednesday, July 31, 2013 5:16:25 PM UTC+1, tim..... wrote:
Just moved into my new flat.

It has DPS "High Security" locks which are (presumably) un-pickable.

However, I have one key which own both the communal front door and my own
flat's door.

I guessing that means that I can't change the cylinder and get a batch of
new keys as they then wont be able to open the front door (unless that is,
there's only one common door keying).

So I'm stuck with having to hope that previous tenants haven't kept a key
(Or worse, given one to their friends).

There are systems which allow the front door to be opened by a multiplicity of different keys, but they can be complicated. The locksmith who fitted the locks in the first place may be able to produce a new pinning for the cylinder which still opens the front door - though often these things are ordered as a complete suite of locks requiring you to go back to the manufacturer armed with permissions signed by anyone who might have an interest and then pay an arm and a leg...
I think I'd just give up on the convenience of having the same key for both doors unless it's something that's ruled out by the tenancy agreement.