This is just a quick update to say that Cloudscape have been in touch and the problem now appears to have been resolved. I'll leave it for them to provide the technical explanation, but it wasn't anything at my end.
Yesterday we ran a test here via Ethernet after rebooting the router that showed speeds were significantly down on expected results and at one point wireless slumped to just 3mbps at around 6.00pm. It suddenly improved quite dramatically at around 9.00pm last night and by midnight it was nudging 60mbps.
Since rebooting the router yesterday the sync rate has dropped to 66mbps, however it's still in the DLM period so that can be looked at over the next few days. The actual download speed with wireless was tested a few minutes ago and I was seeing 62mbps. http://www.speedtest.net/my-result/6152087498
A huge thank you to everybody here who helped to resolve this.