As for removing that first symbol, you could use something like

CASE WHEN SUBSTR({custeventcustevent_initial_cost},1,1) = '£' THEN NVL(TO_NUMBER(SUBSTR({custeventcustevent_initial_cost},2)),0) ELSE NVL(TO_NUMBER({custeventcustevent_initial_cost}),0) END

should i be using two case statements?

You can, I've only nested case statements on a really ridiculous search. In your case, we've had issues like this in the past and I had our project managers take care of correcting their fields. In another case, I did what you were talking about doing, exporting the data from a field for all records that it is not empty, then changing the type of field it is to currency, then re-importing ( after cleaning the data in excel )