The makers specified re-heat time is 79 minutes for the 180l model.
You may have two immersion heaters, in which case the lower one usually runs on a cheap rate electricity, like the Economy 7 tariff. However, this only comes on during the night (something like 11pm to 5am I think).
The top immersion heater is usually designed to give a quick boost, but doesn't heat the whole tank. If a) The lower immersion is switched off; AND b) you like a long shower, the top heater may not warm enough water for you.
The immersion heaters look like white bosses on the outside of the cylinder, about 4inches in diameter and about three inches deep. Each will have a thick cable (usually white) going to a separate switch (usually mounted on the wall, and sometimes with a red neon indicator) or to a timer. Make sure both are switched on and try again in the morning.