Bath shower mixer hot/cold

[jclio96]

Problem was bath mixer going from hot to cold every few mins, I first thought it was a boiler problem but all other outlets are fine, so I changed the bath mixer taps to Bristan, the problem has now reduced to the temperature is staying the same but has constant drop and rise of around 2 degrees, I notice that the flexis underneath are crossed. My next step was maybe putting a check valve on the hot. Any ideas ?.
Additional info-
Combo boiler Worcester junior 28i
Boiler is less than 3m from bathroom.

 

[JCplumb]

You're unlikely to manage a properly constant temperature with a manual mixer on a combi, best bet imo is a thermostatic mixer. They're not cheap but they're a lot better than a manual mixer.
I put a manual mixer in my mums house about 10 years ago, still going strong, but the temp does fluctuate a bit, she has a combi too.
Doubt a check valve will help, your hot water is pretty much mains pressure on a combi, lower flow rate but similar pressure, so wouldn't think the cold overpowers the hot.