I have read that you are likely to spend ages on hold and since these
calls are billed at something like 2 euros per minute [...]
You'll be glad to hear that neither of these are necessarily true.
Firstly, I have called WizzAir thrice (twice in the past month) for customer service related enquiries and each time I was on hold in a queue for less than a minute and the agent dealt with my issue quickly and efficiently.
Secondly, since your enquiry is related to an existing booking, you don't have to call the premium rate enquiry number, but instead you can call the UK local rate number for existing bookings (source) which is +44 330 977 0444. The other way to avoid calling a premium rate number is to call one of the international numbers from the link above and find an English-speaking operator. I haven't tried this myself, but the Malta enquiry number is +356 2778 1054 which is a domestic rate number and almost everyone in Malta speaks English. There are many ways to call international numbers cheaply these days - Skype for example charges 16 euro cents to call a domestic Maltese number. If that doesn't work, there are several other numbers at the link above which charge substantially less than 2 euros per minute.