Due to the thickness of the wires, in the end I could not use the DC plug from Maplin I mentioned earlier.
For those going similar route, I ended up using a solderless 2.5mm plug N53GU from Maplin. Whilst it is for mainly for temporary connections, everything seems very secure and I cannot see them accidentally pulling out even with a tug. The only negative that I see s that it s quite bulky and with thicker cables as from the CTEK unit, you would not want to close the pump bag with the plug inserted, such as to keep he CTEK indicator plug permanently connected.
But there other alternatives. I couldn't be bothered to get the soldering iron out when I could just press the two wires into the socket.