ive ended up getting a copper reducer pipe from b&q, it fits my 22mm aps tubing and reduces it to 15mm, the tubing is really tight but to be on the safe side ive thrown on jubilee clips. and i have purchased a new 16mm spraybar and tubing to attach to the other side of the heater. i am aware this will reduce my flow rate a little but because of the large diameter of the aps spraybar circulation in the tank was poor. im hoping that because ive reduced the diameter of the pipe i will have increased pressure, thus increasing circulation albeit at the cost of a little flowrate.