Thanks again to both for your help. My filters are as follows: Rear, white W2ITSHK710 ans side, orange - W2ITSHK700. So all appears ok based on colour coding (I could not relate codes on Messicks, although it is a very useful website).
New plates all put in with assembly lube, all spotlessley cleaned and put back together with great care and attention but same results (L 5mph, M 7mph H 10mph). I tried pulling the mechanism on the top of the hst box and managed to get it to 15mph...but very puzzled as to why this happened as all linkages appear to be allowing full stroke. In desperation i had to take a full load of rubble up a farm track (1.5miles uphill). It managed no problem although very noisy with high revs! Will try not using it again until fixed properly. Oil levels remain ok with no major leaks detected.i thought i had a leak but think it was all the oil and old oil on to of tranny box when heated up went fluid again!! Need to get power washer out!! Think i will revert to dealer and update them on my work to plates etc. Will keep you all posted on finding. I did try knocking piston but it didn't make any difference unfortunately (wasn't sure how hard i should hit either!!). Thanks again.