Fertiliser for root veg any recommendations?

I would not use any fertiliser for roots. They would normally follow potatoes on rotation, which is a crop that does need fertilisers. Fertilisers can cause lush growth with poor roots and forking.

The Amateur Gardening Pocket Guide – first published in 1941 – the time when gardeners would mix their own fertilizers up for different crops says:

 Root Crop Fertilizer:

sulphate of ammonia...... (nitrogen) ----- 1 part by weight
superphosphate of lime.. (phosphate) -- 4    "           "      
sulphate of potash......... (potash) ------- 2    "           "    

mix well and use prior to sowing at the rate of 4oz per square yard.

(great name you've got there btw