This has happened twice now, in the evening after someone's had a bath or shower, the water seems to 'run out'. It comes back after a couple of hours, downstairs is fine. We do have a tank in the loft, but I'm pretty sure we don't use it now. I've checked the pressure in the boiler and it seems OK, any ideas?

It could be due to the area u live in some places reduce there water supply in summer

Are you in the UK, what feeds water up to your tank, is there a flaot shut off?