@TipTopCloneas Eric mentioned you'll need a large shifting spanner. If you don't already have one, it's definitely is a worthwhile investment - useful ...

It looks like the tap has a brass bush attached. I suspect it's a step-down bush, in which case the answer may be as simple as "remove the bush". That ...

I guess by now you'll have done whatever, but I'll add this comment for the sake of others who might come looking with a similar problem. Check under ...