Vaillant Geyser

Hi,
Apologies for the long post, I am new here.

I have a Vaillant Geyser quite old from the look of things. Over the past 12 months, I noticed that I wasn’t getting very hot water in the shower. However, I played around with the water flow and found that if I had the flow low it would get hot. So I lived with this for a while. Then recently it changed again and I found that when the water first came through it would be hot and then cool to lukewarm, I played with the water flow and it worked but the flow is so low that it makes showering very difficult, not to mention that this of course effects the rest of the taps. I also noticed that the flame from the heater it’s self is over shooting the casing and actually comes out of the square hole where you light the pilot light from. When I removed the casing I noticed there was a lot of black sooty looking stuff over the gas jets, along with this I think I can smell Kerosene or gas when I use the hot water. I spoke to a plumber but he didn’t know anything about these old systems. I am pretty sure it is not the diaphragm as this was changed some time in the last year or so. I would like to keep the system and given that many of these types rely on water pressure I don’t want to go out and buy another and find that the problem is the water pressure and that nothing can be done about it. I hope someone can help. Please try to keep the answers reasonably simple as I have very limited knowledge of plumbing and gas fitting.

Thanking you in advance.