Step away from the Xtreme! And any other Mothercare ownbrand for that matter!!
Some people swear by the XT, but I don't like it to look at and the plastic bits look cheap to me. I've also been completely put of all things Maclaren by the twin Traveller.
If you're happy to have a forward facing seat, the Britax B-Agile would probably be your best bet. You can easily attach a carrycot or carseat onto the chassis if you like. Or a soft carrycot. I would suggest the Baby Joger City Mini, but it's probably a bit tall for you. However, if March isn't too late, the BJCM GT would be much better as it has an adjustable handlebar and air wheels.
If you really would prefer a parent facing pushchair, I'd suggest the UppaBaby Vista. It has everything going for it and you never hear a bad word said about it. They're about £550 and come with the carrycot and Maxi-Cosi adapters.
Another parent facing option would be the iCandy Peach or Bugaboo Cameleon (or Frog or Gecko if you want to buy cheaply second hand). The Peach has the benefit of being able to convert to a double in the future and is a lovely pushchair as both a single and a double. The Cam, Frog and Gecko are absolutely lovely to push, but access to the basket in pram mode is difficult. The Peach, Cameleon, Frog and Gecko all fold in two pieces which can get a bit irritating, but not if you aren't going to be in and out of the car all the time.