For the first time since cheap domestic fares have become flexible (for a fee), I've been trying (since Friday) to change one for next weekend.

I only need to change the return leg (to earlier in the day), so when I choose "I don't want to change the date or time of this flight" for the outbound, and "I want to change the time of this flight" on the return, I get a screen showing all the GLA-LON options colour-coded with an A-E price rating. When selecting the one I want, I wait for a bit, then get a red message saying "We regret that due to a system problem, it is currently not possible for you to make a booking. Please try again later."

If I say that I want to change the time of BOTH flights, I get a non-coloured table showing ALL flights are full (on both days). I know they aren't. In fact, I can book the very flight I want home on ba.com for approx £90 each. Alternatively, I can book with Easyjet for around £60.

Is this a regular problem? Will ringing incur the extra £15 on top of the £30- change fee and difference in fare?