If by 4 months off you mean taking the optional winter leave then those figures seem way off. On the 737 as a DEFO with experience in the UK you would expect basic of £53,916 plus sector and duty pay. The 737 is definitely queiter in the winter but I looked at my friends roster and he is still flying 7 days a month in November so not exactly commutable. If you are talking about the winter leave option then obviously reduce these figures.
For context on the 757 I have 7 days flying between now and December.
As for the Spanish contracts I have no idea where these figures are and I would be interested to see them myself.
None of us mortals knows what the bonus scheme will be exactly, full private health insurance takes effect from January 2018 I believe.