Our water mains have recently been replaced and the water was turned off for a few hours theother day. Since then our cold water storage tand is only filling very slowly. The pressure on the cold water fed directly off the main seems fine, but the tank in the loft is only filling at a trickle. How do I check for a blockage either air or silt? I have checked the ball valve in the tank and that appears to be fine.

Who/where was the water mained turned off? Likely broke loose some debris that is now clogging your valve.

Our water mains have recently been replaced You are talking about a city water main and not a private pump. Correct? What's the PSI coming off the main?
If you have good pressure at the main then shut the water off to the tank and open up the float valve. Check and clean the inlet. Now turn the water back on to flush out the supply, Reassemble and test. Better now? Let me know, Tom