Can poor cold water flow to my shower, which is gravity fed from a tank in the loft, be improved by means of a pump. If so could you estimate the likely cost?

Hi jacksyl and welcome to The Plumbing Page at ANHD.com.
Is all your cold water gravity fed or just the shower?

an poor cold water flow to my shower, which is gravity fed from a tank in the loft, be improved by means of a pump
Sure it can! You treat the tank just like a well with a small booster pump, a check valve and a small bladder tank. To estimate the cost I'd need much more information then you've provided. Get three estimates and go with the one you're most comfortable with. Good luck, Tom