I may have some ideas. Firstly, using this stroker in the bath with Durex lube is not the best idea. Durex lubricant is water based and therefore, water soluble. (It dissolves and rinses away in water, becoming useless) If the bath water is washing away your lubricant, this will cause a lot more friction.
Secondly, it is possible for some strokers to just be too tight for some men, and although you have not explained exactly which stroker you have, (there are 2 Tracey Cox Supersex strokers) I am going to guess you may have the "Tracey Cox Supersex, Super Tight stroker" This one:
http://www.lovehoney.co.uk/product.cfm?p=22208
For guys with larger penises, or problems such as phimosis, really tight strokers can be painful. This is definitely possible and it happens.
Suggestions:
Turn the stroker almost in half, like you are folding half of it back over the other half (Kinda like when you join a pair of socks together) then place lube all over your penis and the stroker, place your head slightly inside and then fold back the stroker over your penis. This might allow you to use it without the extra long initial stroke required to fully insert your penis.
Also try getting a thicker water based lubricant, as this may help too. (I recommend Sliquid Dude Lube H2O or Sliquid Sassy) and also avoid using it in the bath, so you do not wash away, or dilute the lube with steam or water.
Alternatively, if nothing works and it remains uncomfortable, remember that Lovehoney have a great returns policy, so you can always return it and try something else. heres the returns policy info: http://www.lovehoney.co.uk/help/returns-policy/