We get a strong smell like sulphur in the kitchen when the central heating is on.
The boiler is in a small shed outside of the kitchen.

We had this last yar but it went and this year we knocked off our heating for a few days when away from home and after we put it back on the smell came back.

We have checked the gas and drains etc and they are OK.

Any solutions?

This suggests somewhere a real hot (in temperature) wire or contact related to the central heating.
Just check the involved connections and/or outlets for smell and warmth. Once found : tighten up the connection.

Success !

Do you have baseboard heat.fan coil, or
?

Is it oil fired?

It is a gas central heating system

I would have it checked. It could be a defective heat exchanger since it is gas central heating. If it was a boiler as you said in your first post this would not apply except to say that the fumes could come from the burning of the boiler and leaking into the house.Always better to be safe than sorry.