Thanks for your replies everyone, this whole issue has been bothering me for a while so it is great to get people perspective on it.
It is interesting that other people have also experienced something similar. I have never come across this before. Obviously there has always been conversations about diets wherever I have worked but not these sorts of comments or attempts at pushing food on others.
Hatice good tip about the book that would allow me to have a sandwich now and again!
pinkdelight yes I do wonder if I say don't comment on my food and weight she may well think I have an issue when I clearly do not
Preciousbane thank you, I will look through old threads
Plumppartridge I have put cakes and biscuits in the bin because I have been pressurised so much 
dontmindifido good point, I am taking it personally but maybe I shouldn't because it is actually about her not me. I need to be consistent and keep explaining I can't have this or that. People do back off when you give some sort of allergy reason. If I say I dont want a cake or whatever, people say go on, just have one and can be quite pressurising
motherinlawsdung
good idea!
Spermysextowel great name btw. I am not brave enough to say that!
SDTGisAnEvilWolefGenius maybe she is trying to show her love by feeding me but she is saying I should stop eating like a silly scarecrow. I wouldn't and hope I wouldnt treat anyone differently just because of their size.
ArgyMargy that is what I have been doing so far, accepting the food because of the pressurising and then dumping it. I sometimes take it back to the kitchen at work or give it my neighbours or a friend
LessMissAbs the loan is a personal arrangement between my boss and the head of department, his own personal money. That is why it is difficult for me to approach him because he and my boss are very close.
Beastofburden most people at work are happy to eat it or take it back to their offices, maybe they dump it. I have tried bringing fruit into work. I brought a bowlful, it ended up going mouldy because people were not that interested. My boss is trying to lose weight, it is interesting that you say when someone wants to control their eating they encourage others to eat more. She regularly bakes cakes and brings them in, lately she won't have a slice herself though but will happily put a ginormous slab on my desk
Xenadog I do feel sorry for her. She was saying to another colleague that she fears that one day she will get so big she won't be able to get out of the house. I think she is lonely. She has not had a boyfriend for many years
Yellowknife I would have to take it very high to bypass the head of department and I do think it would be seen as a trivial issue. I like my job, I don't think it is worth leaving this job over. I need to think of alternative strategies to deal with it even if I have to make up dietary lies 
livelablove I am fortunate to have a permanent job, so no intention of leaving. Not really any options of transfer. Being where I am is great in terms of fitting in childcare and I could possibly lose that if I transfer
I have been overweight in the past and I mentioned to her that I used to be a size 18 but then I lost the weight. Maybe this annoys her? Not sure.....