can anyone suggest a good external sequencer to get me started on my career of musical fame?
its a big market out there.

ideally somthing witnin £200 range.

i want somthing that can let me try a lot of different rhythmns.

For sequencing a 404? You might be able to get hold of a used MP2K for £200, I saw a 2kXL for £250 last summer, there were some problems with the outputs so it wouldn't make any sounds but the sequencer worked fine. It is going to take some looking and you may have to go slightly over budget. For all their failings the MPCs still have the best, most rock solid sequencers available in my opinion. You won't regret it.

Alternatively there is so much free software available that you could just buy a cheap MIDI interface and use your computer.

Can't go wrong with an MPC. I use mine to sequence external gear (synths, etc) all the time, never lets me down.

Alternately there's the Yamaha QY series. I think there's at least one QY 70 or QY 100 user on the boards. They can usually be found for fairly cheap, plus they're portable! However I can't speak for how user friendly they are or how robust the MIDI programming is.

Totally tangential but I've always wanted an Atari STacy for MIDI duties.

Yep. I've seen a couple of videos of people driving an SP qith a QY. Seemed to work pretty good so if you're looking for a portable solution, that's the one for you. They're cheap but the whether or not the lack of extra functionality is deal breaker or not is up to you. With an MP you get the obvious benefits of it having a sample engine and 16 velocity pads, but on the other hand they're very large in size - I'd like to see you try to make a beat with a 2k on the bus. Some of the newer generation MPs are quite portable but I haven't used any of those and I've heard they're not nearly as stable as pre-4k models). I run my whole studio (SP-404, Akai s950, Novation Bass Station, Pro Tools) using a 60II and it works great.

RM1X? The only thing that annoyed me about is that to switch channels while in grid edit you have to press stop. but that shouldn't bother sequencing the SP because all the pads are on one channel. (wait actually.. its on 2 or something isnt it? hmmm be wary)