Glow Worm 30cxi service engineer left a card

[xfr]

had boiler serviced end of may.the engineer left a card saying further work needed doing.
it reads like this external controls!!High temperature differentials 30+%
what does this mean asked a plumber at work today but he didn't know.
the card got shoved in the draw only just found it today..

 

[Boilerman2]

Is the Boiler working to your satisfaction?? if so
the card is a load of Clap-Trap!!! probably an attempt to sell you a programmable room Stat for £200 !!

 

[xfr]

Yeah the boiler is working better and smoother than ever.
So you think it's room stat related

 

[gas_man_gaz]

could mean the differential in the flow and return temps? why don't ring um?