Check around there with your magnet and obviously check that that gate valve is fully opened by shutting it ( ~ 3 full turns) and then reopening it fully.
What make/model of HW cylinder.
I have a 150L twin coil (oil fired coil and solar coil) the oil fired coil is 0.62m2 and I carried out tests a good few years ago, the coil absorbs ~ 8kw with a dead cold cylinder and falls to ~ 2.5kw (flowrate is 10.0LPM) when the cylinder is approaching its SP temp of 60C, boiler flowtemp is ~ 70C (65C/75C, oil fired cut in/out) this is no problem as oil fired boilers spend their whole life cycling but could present poblems wit a gas fired boiler which fire up at about 60% of their max output and if they can't/don't modulate down rapidly enough then will trip the burner at their SP+5C and won't refire until the anticycle time has elapsed, this may be your problem? especially when the cylinder is getting up in temperature.
What make/model of HW cylinder.
I have a 150L twin coil (oil fired coil and solar coil) the oil fired coil is 0.62m2 and I carried out tests a good few years ago, the coil absorbs ~ 8kw with a dead cold cylinder and falls to ~ 2.5kw (flowrate is 10.0LPM) when the cylinder is approaching its SP temp of 60C, boiler flowtemp is ~ 70C (65C/75C, oil fired cut in/out) this is no problem as oil fired boilers spend their whole life cycling but could present poblems wit a gas fired boiler which fire up at about 60% of their max output and if they can't/don't modulate down rapidly enough then will trip the burner at their SP+5C and won't refire until the anticycle time has elapsed, this may be your problem? especially when the cylinder is getting up in temperature.