I seem to recall reading that the last supplier (they had several over the years) of Jessops paper was Kentmere, but I wouldn't quote myself on that...
What print developer and how long are you developing for and at what temperature? I use a rule of thumb that I leave the print in the developer (constant gentle agitation) for 5-6 times the first emergence time. This ensures full development pretty much. This translates to about 1.5 minutes with MGIV-RC in most developers (but as much as 3 mins in some, such as Ilford Cooltone dev).
MGIV has one of the deepest Dmax of any paper available so if you are not getting a good black, there is something wrong somewhere. If it's more a question of lack of contrast, again, MGIV easily reaches a true grade 5 unlike some papers I could mention, though some colour heads can not supply enough blue light to reach grade 5.
Ignoring negative contrast as you have printed them OK in the past and assuming the paper is fresh(ish), is it possible your head's filters are faded? Try some separate MG filters if you have them and see if that improves matters.
Another possibility is the safelight filter may have faded or you have moved the safelight too close to the paper, Slight fogging from the safelight, not otherwise obvious, could lead to you underexpose the paper to avoid the highlights going too dark, but not leave enough exposure to get a good black, even at grade 5. Try a print without the safelight and see how it goes.
Good luck.