I recently purchased Maplins leadfree solder and had lots of issues with it. I thought it might have something to do with the soldering iron not getting hot enough so bought a new Antex iron. The old one was over 20 years old.
Unfortunatly the bit on the new iron became very black very quickly and again refused to melt the solder, so I bought new tips for the iron.
After doing some research I found other similar complaints about Maplins own brand solder, so ordered up some Multicore Savbit Aloy Solder from ebay.
What a difference it makes. It melts quickly and flows easilly.