no i think iam the only one that has recived full sick pay but not sure

hi new here can anyone give some advice please i had a week off on the sick (had sick note) my wife collected my pay slip and pay i received full pay for that week. went back to work at the end of the week (Fri) i did all the wages for the staff and mine as normal but no pay slips were sent (our contract states pay on last working day) so i paid my self full pay. so on Sunday i called into work and the pay slips were on my desk my pay was £29.ssp and £20 tax rebate but for the last five years i have received full sick pay but in my contract it does state ssp only. so i phoned my boss on Sunday and asked them why i only have sick pay they told me that is was stated in contract. i told them that because they paid me full pay for the last 5 years that it is part of my contract. i phoned again on Monday and was told i would get my full pay at PM i received a phone call from a personnel manager telling me i would not get my full pay and my contract was void? and told me i had to get proof from my doctor of every sick note Ive had. the doctor told me he does not keep records of sick notes. so on Tuesday i brought up a grievance with my contract on Thursday they offered me a one off payment not for sick pay or overtime but for all the hard work i had done last year and to think about it. and i will receive a letter in a couple of days. i have not decided to except it but i had a note attached to my pay slip saying the payment will be in next weeks wages. i can not carry on working their because of the way they treat me after 5 years service i already got another job at the end of the month but i will lose £5000. a year