I forgot to come back and update. After speaking with Thule I learnt that a 753 or 751 footpack is fine (one is taller than the other), however a gap between the footpack and the car roof is "normal". Apparently the gap can be up to 2 inches! Which in my honest opinion is totally unreasonable and looks stupid!
We returned to Halfords (who fitted our bars) and they agreed to take them back and give a full refund. We then bought proper Renault ones on
for almost half what the Thule ones cost - and they were brand new!!!!!! Ironic really as last time I'd priced the Renault ones they were more expensive so I just bought the Thule ones without checking the price again! :D
So now I'm happy with nicely fitted Renault bars. As for Thule, - well their customer service left a LOT to be desired and I won't be buying from them again!