A 70-year-old man presents with painless haematuria. Cystoscopy reveals a papillary tumour on the bladder wall. Biopsy confirms high-grade T1 transitional cell carcinoma (TCC). Staging investigations show no muscle invasion or metastasis. According to NICE NG2, what is the most appropriate next step in management after initial transurethral resection of the bladder tumour (TURBT)?