Since the type was known i assume the controller had some info on him. If he used the mode C he must have.
For a 7000/1200 unknown I would use "unknown traffic passing x to y indicating 17500ft unverified, will pass directly in front of you, suggest 15 degrees to the left for avoidance" or some such.
There is quite a few countries where class E effectivly becomes D at night, and you have to seperate VFR night from IFR due to a "all flight at night in class E airspace must be performed according to the instrument flight rules" clause in the national version of Annex 2.
VFR Night in class E = IFR = Need to separate
I´m sooo glad the last non offshore/int. waters class E in our FIR is going on some airac date this spring