Question to the Home Office:

To ask the Secretary of State for the Home Department, how many thefts of catalytic converters from vehicles the were in each month in 2018; and how many of those thefts resulted in (a) an arrest and (b) a conviction.

The Home Office collects data on police recorded crime including theft from a vehicle. These are published quarterly by the Office for National Statistics (ONS). The latest figures, for the year ending June 2018, can be accessed here:
https://www.ons.gov.uk/peoplepopulationandcommunity/crimeandjustice/datasets/crimeinenglandandwalesappendixtables

However, these data do not identify whether or not the theft is of a catalytic converter. The Home office does hold any data on how many thefts of catalytic convert-ers resulted in an arrest or a conviction.