If the battery is smaller than the battery tray, it's definitely not the right battery. You need a heavy duty calcium silver battery for your Mondeo - diesels take a LOT of power to start.

[edit] Hmm, seems I already mentioned that in this very thread two week ago: http://www.fordownersclub.com/forums/topic/52707-mk3-20-tdci-starting-problems/#entry332026

Indeed you did! Sorry my mind has been all over the place the past week or so!

I have undone all the battery terminals and cleaned them and greased up so see how things go. I think the battery is the wrong type as it is taller and not as wide. I noticed in the scrap yard the 06 Mondeo had a nice Bosch battery that took the full width of the battery tray

Maybe, make the scrappy (this is the relatively cheap scrappy that has supplied you with starters?) a silly offer for their Bosch battery, and see how they react? Maybe if you mention the general aggro you've had with their starter motors, or even offer them a starter back in part exchange, they'll be sympathetic enough to offer you a better deal.

(Even if you go to Halfords, which I don't really recommend for purchases, you'll see that batteries are a bit variable in dimensions, for a particular capacity, with one manufacturer making them slightly bigger than another. If it doesn't fit with the 'tie down' bracket, there is definitely something wrong.)

Is there any sign of a label on the side of your battery with 'ampere-hours' or 'cold cranking amps' on it? If both of those are ok (70+ AH, and the more CCA the better) the car should start (if it isn't a 'Calcium' Lead-Acid battery, the charging system may be a bit over enthusiastic for it, but that'll contribute to a shorter battery life, rather than preventing it from starting in the short term.)

Update: another starter had packed up this morning! So this one lasted 2 weeks! I don't know if it's worth putting another starter on or just get rid? I can't afford to put it in to a garage