Retrieve attachment request returns "Request Entity Too Large" with no further details

I am trying to download copies of particular form responses using webhooks by Zapier.
The GET request is sent to https://api.servicem8.com/api_1.0/Attachment/[UUID of the Attachment].json

Approximately 50% of the time this works without any errors but the rest of the time i get the error message "The app returned "Request Entity Too Large" with no further details. This usually means we were able to connect to this app or service, but it indicated a problem with what the Zap was asking to do. This could be related to the connected account having a changed password or limited permissions or with necessary values for this step being missing or in the wrong format."

I have contacted ServiceM8 Support here http://support.servicem8.com/hc/requests/72609 but they seem unwilling to help find the issue. The response i had from them was "413 error means the URL is too long. So this is what needs to be addressed to have it fixed" but cannot tell me which URL is too long?!

Can anybody please help point me in the right direction?
Cheers
Sam