Phil,
I'm no longer registered with OISC so I don't get updated on new stuff anymore.
But I believe that a replacement 30 day Travel Vignette is dealt with under Transfer of Conditions
The cost is around the £70-£90 mark
Your letter will detail.
Give or take a little time it shouldn't be too difficult to manage unless there are some time consuming issues to take care of. Most folks on here seem to fly to UK fairly soon after the visa decision.
It does mean compromising on short-time decisions but using the UKVI processing times to determine a reasonable travel date should work out reasonably well.
Hopefully