I have a few holes to drill and so does a friend so we bought a 127mm x
150 diamond core drill between us, it came with a 200mm sds+ core drill
arbor and a 10mm x 200mm tapered guide drill.

I bought a Titan 1250watt sds+ drill with safety clutch from Screwfix
and today attempted my first hole through single skin brick.

I first fitted the pilot drill to the arbor and drilled that hole right
through, then fitted the 127mm core to the arbor and attempted to start
drilling.

What a mess, it was just like trying to drill the core out without a
guide drill fitted at all. The 10mm bit only sticks out 15mm in front of
the core and after just a few seconds that had opened the pilot hole up
so far that the likelyhood of ever getting the core to stay in one
circle is remote to say the least.

What did I do wrong?

My thoughts are that the pilot drill as a guide with just 15mm
protruding is just not enough to guide the core.

I understand that once the core is into the brick by 20mm or so the
guide is probably best removed, but how to get to that stage!

Having tried it I would have thought a 13mm x 250mm or more guide drill
would be the ideal to keep everything going neatly until the core is
deep enough.

Suggestions please.