are traders directly involved in negotiations to set Market & Credit Risk Limits for Trading activity ?

any input ?

IMO, Yes at least for Market risk limits such VaR and greeks.And usually done once or twice a year by head of desk.

what is their likely input and arguments ?

They ask for limit sizes that are in accordance with the business plans and target revenue. Ie, if they plan to take large client orders they may need larger risk limits to allow time to efficiently hedge.They also provide input on whether they can feasible operate their business under certain limits. If not, and the business is desirable, negotiations with risk management may ensue.Also, since senior management of the trading desk should have an incentive to not blow up, they may suggest limits that otherwise would not have been considered.Ideally this is a collaborative process.

at what point in the negotiations process is it considered that the front office is "influencing" the process of setting limits ?do they have a say on the various methodology's used in quantifying Market & credit Risk ?

Very strange question. Traders are the ones asking for limits so of course they will be part of the negotiations. Are you suggesting risk management negotiate with themselves?