What type of Acer? Many varieties have bark with white markings as part of their ornamental features....could you supply a pic perhaps?

Im not sure what type of Acer it is but Ihave had it for 8 years and this is the first time I have seen this, on closer inspection the white is a pasty substance and there is brown lumps on some of the white stuff.

That sounds like Scale Insects - they are small sap-suckers that create a brown "scale" as a protective shield with fluffy white substance beneath and behind the scale, quite common on the trunks of Acers and other trees, particularly in Urban & suburban areas. Once they have attached to a plant they cannot move so if you only have a small-ish number just rub them off with a soapy sponge, being careful to collect them in a pot/bowl etc - the white bits contain egg masses too, so try to get as many as you can.

They are highly resistant to most insecticides, but can be killed with fatty-acid based sprays if you have a very large infestation.

Thankyou I will give it a go,

Mosely