[Cobbydaler]

From the PIL for methotrexate tablets:

Taking/using other medicines
Special care is needed if you are taking/using other medicines as some could interact
with methotrexate, for example:
 non-steroidal anti-inflammatory medicines e.g. ibuprofen (medicines taken for
pain relief)
 aspirin or similar medicines (known as salicylates)
 medicines taken to help control rheumatism e.g. leflunomide, azathioprine and
sulphasalazine
 omeprazole (medicine used to stop the production of stomach acid)
 diuretics (water tablets)
 medicines used/taken for diabetes (including insulin and tablets)
 antibiotics such as penicillins, sulphonamides, co-trimoxazole, trimethoprim,
tetracycline, chloramphenicol and para-aminobenzoic acid
 phenytoin (medicine often used to treat epilepsy)
 vitamin supplements containing folic acid
 probenecid (medicine used to treat gout)
 nitrous oxide (used for general anaesthesia and pain relief)
 retinoids, such as acitretin (a medicine used to treat psoriasis) or isotretinoin
(used to treat severe acne)
Please tell your doctor if you are taking or have recently taken any other medicines,
including medicines obtained without a prescription.
Taking Methotrexate Tablets and drinking alcohol
Alcohol increases the risk of liver damage with methotrexate and it should be avoided
during the full period of treatment.

From the PIL for dithranol:

Using other medicines
If you have been regularly using topical corticosteroids
to treat your psoriasis it is important that you allow
yourself a treatment-free interval of at least one week
before starting to use Dithrocream. You can use a
plain emollient (skin moisturiser) on your skin in the
meantime.

So looks as if it should be OK, but you should really check with your doctor. (Although he'll probably use the same or similar source that I did as well as his experience).