Changes in temperature

My set up is currently in the airing cupboard, the temperature changes as it would only get warm when the heating is on or when water is being heated, does changes in temperature affect the taste at all? Should I invest in something to keep it at a custom temperature?

What cheap effective ways are there? I've seen someone use a reptile heater, are these any good? What would the set up be like?

Controlling fermentation temperatures is one of the keys to making better beer. And the yeast you use and what you are tyring to get that yeast to do, from a flavor perspective, will determine the temperature you need to hold. For my brews that use yeasts that require higher temperatures, like saisons, I built a box (without a bottom) out of rigid foam used in the building trades. I place the box over my fermenter and use a ceramic reptile heating element in a light socket to do the heating. It works well. You will need a temperature controller for this to work, but you can build one using an STC-1000 and some electrical parts from the hardware store for under $30. For cooling an inexpensive way is to use something like this from Cool Brewing - https://www.cool-brewing.com/. You drop your fermenter in along with some frozen water bottles and keep temperatures down. If you have a stick-on thermometer strip on your fermenter, you can monitor temperature and adjust the number of ice bottles to maintain the temperature you're after.

Once you have your fermentation temperatures consistently under control, you'll love how your beer tastes.

Good luck.