Bosch caravan boiler W135 won't light?

It is an old boiler/heater, over 10 years old. We have just turned it back on after the winter shutdown and find the pilot light won't light. Water is on and fills boiler. Gas is available and gets to the other appliances OK. Spark is good but cannot smell any gas when attempting to light. Have had caravan for 7 years and never had this problem before.

It is a Bosch W135, not the type that has a battery to assist lighting.