Hot heater/odd temp reading worrying me

[droptop]

Yesterday I left home with a stone cold 1978 Beta spider.
Within five minutes, the heater was putting out a huge amount of heat similar to my wife's modern (to me) Lexus.
This was strange in itself as the heater has only ever been lukewarm in my six years of ownership.
Once I got on the motorway, things got stranger.
as the engine speed hit the 5k mark, the temp gauge on the far right of the dash, above the oil level gauge, started to climb and hover just below the red zone and the one in the centre below the oil pressure gauge, also went past the point at which it normally sits, a little less than halfway.
I first thought that maybe the thermostat was stuck, but as I slowed down the temperature satarte falling back to normal.
I drove around 3k rpm for a while then accelerated once again, and once more the needles went up.
The heater continued to provide the best performance of its life and when I got to my destination, i checked the fanbelt, which was tight and as the battery gauge was reading properly throughout the trip, I didn't really suspect belt tension anyway.
I let the engine cool down for 45 minutes and checked coolant and oil levels, both of whcih were fine, and after about four hours, I headed for home.
Again within a few minutes of starting, the heater was belting out heat like never before and the high rpm/high temp equation repeated itself on the homeward trip.
Now, the only changes in the six years were the fitting of a replacement top water rail with one that connects to the heater hose instead of the hose connecting directly to the head, which was done three or four years ago, and the topping-up of the coolant yesterday prior to departure, by about 300ml. which is the first time I've topped it up since fitting the replacement top rail.
No escape of coolant or the usual signs of overheating were psesent, the system isn't pressurising and no coolant is circulating in the header tank, in fact it was only the high reading of the gauge coupled with a working heater,  that has caused me to worry.
Any thoughts?

[Neil-yaj396]

I think this is just one of those things that you are going to have to monitor. No coolant loss (although you had topped it up) plus lack of pressurisation seems to rule out the obvious one of head gasket failure?

Could a heater hose have collapsed or become blocked?

[rossocorsa]

I thought beta heaters were always hot but that doesn't explain the guage readings

[droptop]

I think this is just one of those things that you are going to have to monitor. No coolant loss (although you had topped it up) plus lack of pressurisation seems to rule out the obvious one of head gasket failure?

Could a heater hose have collapsed or become blocked?

I'd actually checked all the hoses and the heater hoses are new so I'm at a loss.
I'd topped it up as I'd never actually done so after changing the top rail so it was just something I'd omitted to do at the time, not because of unexplained loss over a period

I think this is just one of those things that you are going to have to monitor. No coolant loss (although you had topped it up) plus lack of pressurisation seems to rule out the obvious one of head gasket failure?

Could a heater hose have collapsed or become blocked?

As above.
No collspase or blocked hoses and it wouldn't be the first odd occurrence with this car.
I had an incident in 2011 when after changing the oil and restarting the car the following morning, I thought the oil had disappeared as the engine noise was horrific and I couldn't see any oil on the dipstick.
I figured the engine was shot so I restarted it again and it was perfect and I had the correct level of oil on the dipstick.
Maybe it's something to do with the reg. number (666 FZB) Spooky!
Such is life with an Italian car.
I'm going to wait for a dry day and try again after changing the sender and see what happens.
I only hope the heater stays working as well as it currently is.