I have to produce some technical documents for work and I keep writing, as an example, 'there is no requirement to .........' and later when i go to edit, I change it to 'there isn't any requirement to .......'
Am I right to keep replacing 'no' with 'isn't' or am I being too fussy?
It's not a question of being fussy but rather one of how formal the work-situation you are in is. As has already been suggested, real formality in writing would demand that you avoid all abbreviations such as 'isn't'. On the other hand, if it is a small enterprise where you know everyone and they know you, it wouldn't matter as much.
In general terms, I would go with "There is no requirement to..." but - in an informal setting - why not just say "Don't..."?
shortened forms like 'isn't' are more common in spoken speech than written. But it depends how formal you want to be. If it's okay for the documents to sound chatty, then 'isn't' is fine. In fact you could even say 'You don't have to...' rather than 'There is no requirement to...' But if you want it to sound formal, then stick with 'there is no requirement'