Hobby Caravan Entrance Door Lock?

Hi DIY gurus,
The lock on my fathers newly aquired Hobby is not working. The key fits and turns but the mechanism does not lock the caravan door sowe cannot secure the caravan. Ive tried numerous times with WD40. A local dealer estimates £150 to order a complete new lock and says we should be able to fit it ourselves. Ive seen some Hartal locks for around £30 on Ebay, does anyone know if they'll fit ? Also, does anyone have any ideas on fixing the existing lock please?

In addition, the external toilet cassett door is locked without a key ( the main door key doesnt work). We need to get into it causing minimum damage to the door. Any good ideas please? Thank you again, J.