Joz,
1) Have you called your ISP(s)? What did they say? Have you tried cable swapping with known good cables? What about checking any ethernet wall ports, patch panels, all related cables etc. for loose connections. Inside and outside.
2) What modem and/or router are you using?
3) How many wireless and wired devices are on your network?
4) Are you using the wireless security functions? Can you see all of your network devices and only your network devices on the network?
5) Any sense of how many other wireless networks are in your area?
6) Is limited connection occuring randomly or with perhaps some pattern? Just during "day" or including night (i.e., 24 x 7).
Try to avoid changing things and see if you can start a methodical approach focusing on one possibility at a time.
Overall look for overuse or interference. Check the modem/router logs.