Normally at this stage I would have found someone else to buy tickets from -- on the basis that if a company can't manage to produce a simple web page which works it's unlikely that their order fulfilment software works either -- but airline ticketing has worked OK since the 1960s so it's unlikely that Opodo have managed to foul it up too badly.
It turns out that their site, rather than having a phone number on it, has a link you can click which instructs an Opodo employee to telephone you. So I clicked that and an they did indeed phone me. So I explained that I couldn't make a reservation using their website; the lady to whom I was speaking didn't offer any comment but went through the whole booking procedure by asking me what I should put in each box. (``Is it OK for me to click the box that says `I accept the terms and conditions'?'') Needless to say, this was pretty tedious, but it got the job done.