So the people of the UK are going to have a referendum on the EU. And possibly more than one, which poses the issue of the plural of the word referendum. In the various articles I’ve been reading about it, I’ve seen both referendums and referenda used. I instinctively prefer adding the mark of the plural (s) to this word, even if its plural was the form in "a" according to Latin grammar; the word is still used within the context of English grammar and therefore, I think, should obey its rules.
This is what the Oxford English Dictionary has to say on the matter:

In terms of its Latin origin, referendums is logically preferable as a modern plural form meaning ballots on one issue (as a Latin gerund referendum has no plural); the Latin plural gerundive referenda, meaning ‘things to be referred’, necessarily connotes a plurality of issues. Those who prefer the form referenda are presumably using words like agenda and memoranda as models. Usage varies at the present time (1981), but The Oxford Dictionary for Writers and Editors (1981) recommends referendums, and this form seems likely to prevail.