Immersion heater takes a long time to heat up for a shower?

My immersion heater is 15 years old and is taking longer and longer to heat up. From cold, I used to have enough hot water for a shower after 60 mins. Now the hot taps are scalding hot after 30 mins, but for a shower it needs to be on for at least 24 hrs. Even then, it might just be luke warm. The tank doesn't feel warm to touch like it used to. The thermostats are set to max (60 degrees), only the top one seems to 'gurgle', the bottom one has been replaced. The power shower it feeds has not been changed for 10 years.