Hi Rob, depends on what you think's the problem with the current set-up? Too harsh, too woolly, too bassy/not bassy enough, sounds too thin, the sound doesn't project enough/projects too much....and so on!
What kind of sound do you like (as opposed to the sounds you play I mean); detailed, bassy, brighter or a bit warmer sounding...?
Upgrading the amp or CD at your current budget might be more of a sideways step, however, Marantz new 6003 series amps and CDs around the £300 mark (each) may help. Alternatively, there's no harm adding a DAC (such as the Musical Fidelity V-DAC - roughly £150) to your current CDP and using it as the transport. Then again, there's also the secondhand market and you can do well here either via Ebay, or the like, or going down the dealer route, many of whom offer a warranty even on used gear.
All depends on what you are looking to change and achieve as a consequence.