Well, it depends on what version of the milling head you have. The earlier geared-head version (BFE65) came with a piddling MT1 spindle hole plus an MT1 endmill holder that could be secured via a 6mm drawbar. I believe a later version of the milling head (belt drive?) came with an MT2 taper spindle.
So, first you need to establish what MT your version has. Using calipers or a rule, can you measure roughly what size the spindle hole is where it meets the spindle flange? Roughly will do but measure the size at the lip of the actual tapered hole, not the edge of the bevel. Report back the measurement and we’ll tell you what MT it is.
Given the age of these machines, you’ll have trouble getting hold of the maker’s endmill or collet holders now. Nowadays it makes more sense to use ER series collets/holders for these small machines — these are pretty much a ‘standard’ in Europe and elsewhere (except the US!). The maker’s recommended milling maximum diameter is 16mm so a set of ER25 collets would be suitable (range 1.5 — 16mm). Assuming your spindle is MT1 an ER collet chuck can be had for less than £20 — see here, for instance:
But don’t order anything till you’ve reported back!
Joe