I've had "issues" with buying cheap unbranded long tubes from high street outlets so was a little nervous. I found a place on-line http://www.lampshoponline.com/ and was really surprised at how cheap they were. Because I've had problems before, I bought four branded tubes (£2.48 each) and a couple of replacement starters, just in case.
The tubes I bought were Philips T8 Triphosphor - Colour 865 Daylight which were the same specification as the existing ones - though the original ones seem to be unbranded.
After replacing the one faulty tube and switching on it was light looking into the sun. I couldn't believe how bright the new tube was compared to the other one. I'd never really thought about if tubes fade over time and just assumed they eventually burnt out. Anyway, I replaced the other old tube and I'm very impressed with the amount of light being produced. I'm wondering if the original tubes have been fading over time and I never really noticed.
So, don't know if this was something odd with the old tubes, or if they really should be replaced periodically but I've put a note in my diary to replace them next year. Since I bought four and they are probably from the same batch, next year I will replace just one tube with one of my spares and compare the light output.