UltraSteel idirect not keeping water hot?

Hi, I'm a newbie here looking for some help. I have a 300L Ultrasteel indirect cylinder, water heated by Valiant condensing boiler, it was installed about a year back. All has been fine, but I've now noticed that it a lot takes longer to heat water, and water does not remain hot as long as it used too?
Boiler seems to be working fine, no drips or leaks from the cylinder, no pressure drop, any reasons why this could be happening and how it could be fixed?
Manual says annual servicing, I'm hopeless at anything diy so don't want to screw anything up by trying it myself.

Any advice would be appreciated.

whats do you run HW on, timed, contant etc......if its timed then for how long. Bear in mind in the summer months the boiler only had to heat up the hot water tank so no that its winter and you are usuing heating, the tank will take much longer to heat up to temperature......especially 300liters of water.

"water does not remain hot as long as it used too"

Is the tank in a habited part of the house, or a cold cellar?

Is it well-insulated? And the pipes?

Normally on timed, an 1+ hour in the morning with heating, and 2-3 hours in the evening with heating, and sometimes an hour on its own. I have noticed that water goes to the expansion tank as I can see the flexi pipe connecting the expansion vessel to the cylinder moving, and a gurgling noise when hot water is turned on sometimes.

Housed in the garage, pipes are insulated.

I think you may need your hot water on for longer than your are running it......1 hour in the moring with heating is not going to heat up a 300litre tank.